I agree - get your CV out there. The jobs market for IT seems to be fairly buoyant at the moment. If you'd been public sector, I'd have said, that seems to be the way it is at the moment, but IT is mostly doing okay (though it does depend a bit on particular skill sets and roles - I'm a unix sys admin.) There are flexible roles in other companies.
Find out what the going market rate is for your skills/location/experience - use a site like www.payscale.com/
It might also be worth looking at www.glassdoor.co.uk/index.htm - however, going by my own employer, you definitely need to take some of it with a pinch of salt, and it's also quite dependent on department. But it may give you an overview and also a view on salaries.
As AnyoneButAndre says, if you can find someone confidential to talk about salaries, i.e. if they've had recent payrises, then that would be very helpful, but take care with how you handle this - IME, managers rarely take kindly to becoming aware people have discussed salaries.
Do you know how your pay cycle and so on works? Ours are usually done annually, announced around the end of January, so now is definitely the time to start the conversation, because with us, it's when they're doing next year's budgets. In theory, it's based on performance review, although there's still only a limited pool per department, so if everyone is working well (as high-performing teams will do, as they will boost each other's performance), then you can't give them all the maximum possible amount. But in summary, we get something like 1% if people are performing, and 2% if they're exceeding expectations, and I think under performers just get an improvement plan (don't actually know.) Those aren't the exact figures, but it gives the general idea. If you know what the average payrises have been over the past few years, then work out what your pay could be now - e.g. 1% every year, or 2% every year (it's probably been more varied than that.) Where would you be now, if you'd had those rises each year?
Build a case based on market value, what you bring to the department - "I haven't had a raise in years" probably isn't enough, unfortunately. How are your performance reviews? It sounds like they should be okay, and the grade rise with no payrise should certainly be highlighted. Plus you wouldn't get a grade rise if you weren't performing well. Have you done anything that has saved the company money/made more money for the company/improved efficiencies (and therefore had an indirect financial effect)? That sort of thing is always useful to show how you're worth money to them.
As a woman in IT, there's a strong chance you're in a minority, and there could be an equality issue. If they don't just agree you've got a great case and cough up, then you will need to consider going down the official route. There have been quite a few changes to workers' rights with the last government, mostly around the processes involved in going for tribunals and so on - reduced timescales, more costs for the person involved, and so on. It's also changed some of the equal pay stuff - you go through an equal pay checklist rather than an equal pay questionnaire, and while I assume it covers similar stuff, I don't know the details - you need someone who knows.
Look at the 2010 Equalities Act and the 1970 Equal Pay Act in particular. The ACAS and CAB websites will probably have relevant information, too. If you're in a union, talk to them. (If you're not in one, join one, even if it's not recognised in your workplace - they will have the knowledge for cases like this.) Does your company have any other sort of employee reps, or workers' reps, or employee groups like a women's organisation?
It can be worth pushing for - a while back, over two or three years, I had 6 monthly above-average rises to level me up to my colleagues. And in a previous job, they did a pay audit on the department, and I got a 26% rise (having previously been told that discussing pay was a sackable offence, end of discussion, when I'd raised it. Gits.)
Be prepared to negotiate - know what your lowest acceptable rise is, and start higher, so that you can accept a lower offer than you started with. From what you've said they've been like, I would have every expectation that they'll try to fob you off with more promises of doing something which never comes to pass. Ask about redundancy, start looking for other jobs, alongside pushing for the payrise and inequality.
Finally - most big IT companies have policies around diversity and equality and go on about wanting to increase the numbers of women in STEM careers. If your employer does this, then do point out that not giving women payrises is one reason women end up leaving IT. (And all the crap low-level every day sexism, not to mention the overt less low-level sexism etc.) If they say they're in favour of encouraging women, call them on it.