Hes paid more than me for the same job

My goodness, OP, thats a big gap.

You should ask for parity with your colleague. Or say "I'd expect to be on at least the same as colleague, if not more because I do more of the leadership role". Then it's up to them to explain why they think differently.

So you have been there for 1 year? Ask for 35k, then if they don't agree, settle on 33k. If they ask why, say your colleague is similar to you (and you have more responsibilities which would counter balance his PHD research). Say that by year 2, you will expect to be on 42k like him.

If they refuse, start looking another job. If you find one and they accept you, you can either go to that job anyway, or you can tell him you have an alternative job offer an allow him one last chance to pay you more. Please note though, that if you do that you might be staying with an employer who might be nice to you when you're around them (no wonder, he's getting you for cheap), but who doesn't really value or respect you. Even if he agrees to pay you more, it might sour relations between you as your main value over a male colleague was the fact you were female = cheap, hardworking labour.

Are you and he hired on same job title or are they seen as different jobs?

Especially if different job title (and no job evaluation done) as per others I would look to gain parity with previous person in your job. I would ask about how you get to that salary and agree steps to get there including any more training, development that you might need

Pay will mainly be related to how hard it is to replace you, rather than things like experience, etc.
I do know that PhDs, especially in STEM are highly in demand and have been attracting high salaries in my work area, due to the recent boom in Machine learning and AI. They are hard to come by now, so will be worth more due to that.

The fact you are getting better results than the last person is what you should focus on. Do some research first, ask acquaintances in other companies and search job sites for equivalent work to get a better idea of your true "worth". They are probably just paying less because they can and it's never come up.

As other posters have pointed out, women are less likely to ask.

Just ask for your pay to match his in line with the Equal Pay Act as you do 'like work'. They could say that his qualification and additional time in the job/industry experience are material factors that enable him to be paid more, but the current gap in salary is not defensible.

A lot of women in jobs like yours get paid less because they don't ask or negotiate properly.

I think there are some excellent suggestions here, but I know things can get a bit stressful in salary negotiation meetings, especially if you feel there may be an element of basic unfairness at play. Why don't you make a list of all the best points that PP have given and then ask a friend to role play the situation with you, being as obstructive as possible? That way when it comes to the real thing you'll be able to justify your pay request smoothly and confidently, and whatever the outcome you'll know you've given it your best shot.

I get what you're saying about the union but I don't think it's a hostile manoeuvre to speak to your union. If you're a member this is precisely what you pay your fees for! The rep's first course of action will be to help you to work out whether there's a job grading system or some other means of establishing what the rate for the job is. They won't be looking to call a strike or take the employer to court. They'll want to help you to resolve things fairly and amicably.

I work in a field that gives large pay bumps for degrees. My first job was also at a firm that started people on low salaries, but in the first few years gave massive increases. So someone with a degree and an extra year Could have that kind of gap. And you would expect to get much closer in a year or two when he levels off.

But we don’t know if it is that kind of company.

I work somewhere where PhD in an evolving space gets valued a lot more, because a younger degree-only person they can very easily substitute.

There is definitely no harm asking, but also be prepared to walk if it doesn't go your way. You want to work somewhere that pays your worth anyway.

I would use your predecessor as a starting point especially if you are getting better results. If they mention the PhD you can highlight the complimentary skill sets and areas where you have greater responsibility.

I would be asking for £45-48k with a view to close to parity with your colleague.

Probably not worth worrying about until you find out what your raise will be. If it’s still such a large gap I’d ask about it. But I don’t know anything about this kind of thing (due to circumstances I don’t work and certainly wouldn’t be able to do a professional role.)

One more thing I've just thought of - apologies if it's been suggested already and I've missed it. Do you have any idea/could you find out what your colleague started on? Ie is it possible they do lower 'starter' salaries so they can see how people do, and then go up quite a bit after the first year?

This is quite thought provoking:
www.equalitytrust.org.uk/equalpay50/calculate-your-potential-lifetime-earnings-loss

I wonder whether you should either pitch it as £48k, what the last person was earning but you’re giving better results so it’s warranted.

They’ll want to negotiate, so you may not get that figure, I’d then settle for £42/45k.

And I still stand by my earlier point, your company should be fixing this as a priority, not fixing it because you have asked. They are facilitating a gender pay gap, and your loyalty (which is nice) might be blinding you to what is nothing other than basic sex discrimination.

I would recommend you start high with your opening bid and be prepared to climb down a small bit (reluctantly). And also bear in mind PhD guy will probably also get a rise this year. So you will be constantly playing catch-up if you accept a lower salary.
I'd start at 45k if I were you, then let them try to tell you why you should be paid less than him, if they won't settle on equal or more.
Also, in these situations I find it helpful to think, what would a guy do if he was in your shoes and/or roles were reversed? He wouldn't settle for less, that's for sure.

my boss has agreed I need a salary rise, the review is due soon, I am just not sure how much I should push for and want is reasonable to expect.

If you get on with your boss and they know you deserve a rise, you might want to put the ball in their court to some extent. Try to frame the discussion from the outset in terms of having an idea what your colleague is on, and that although he has a PhD you think that's balanced by your leadership role and prior experience. You might need to get those thoughts in play wel before the actual review meeting, as your boss will presumably have budget constraints so can't just give you what you want even if he'd like to. S/he may need bargaining chips to argue your case.

Nothing useful to say, but just wanted to give you a vote of support in this process, and keep us updated !

Are you a union member? I would seek union advice.

I'm aware in comparison with colleagues and others in my role I'm relatively less well paid. I have a year's experience now, and have performed well. I'd like to see a significant increase in my salary, to bring me in line with colleagues. What do I need to do make this happen?

I have worked in places where someone (men and women)has been there longer and is more highly qualified than me. It particularly happens in research science, where pay rises happen on a yearly basis and there is a clearly understood pecking order - no degree, BSc, MSc, PhD.

Well when you and the union have worked it out if you've got the balls I'd say something along the lines of

As X is in a role that is largely supporting my work, and in view of my results I think it would be reasonable if my salary matched his with a view to a further increase if the results (predefined) merit it.