Oops, posted too soon!
The thing is, it’s not that she’s on £10k more than you - it’s that her salary is more than 50% higher than yours, when you’re not only doing the same job, you’re essentially her manager and even though she’s new it doesn’t sound like that will change any time soon.
I don’t know what the industry is so it’s possible there are fixed numbers of roles at each level / band. However, since you are essentially doing a more senior role in all but name and wage, they clearly need another senior person rather than your more junior role.
The rural thing is a double edged sword because while you may be trapped, they sort of are too - it will be much harder for them to recruit for your role, and more expensive because they won’t get someone of your skill, training and competence for what they’re paying you. And they know it.
You’ve already had some great advice from a PP a few comments up and I would handle it the same way. However, I’d also be looking at market rates for the role you’re actually doing (and for the one you’re contracted for as well) and ensure these form part of your discussions.
For example, given the qualifications, experience and training you now have, you could get an extra £Xk if you went elsewhere, along with a more senior role and better career progression. It’s not just about what you’re doing now - it’s about what chances for progression you have in the future. Be clear that this is not the path you were sold when you joined the company.
Personally I’d be making it very clear that they’d be promoting me when your colleague takes maternity leave and that it will be a permanent promotion (if that’s at all possible in your industry) regardless of your colleague’s return, since you’re already doing the more senior role.
(If your role directly generates income this is even easier to argue)
In the meantime, you’re putting in far too many hours and providing too much expertise for someone of your level so tell them they either need to be paying you for the job you’re actually doing, or paying you overtime, or they need to start prioritising your workload so that you’re not doing 1.5 jobs until the colleague’s maternity leave (at which point will they hire cover, or expect you to do 2.5 jobs because they’re being oh so generous in promoting you?).
It’s time to get tough but present it in a way that makes it clear you want to stay with the company, you appreciate their investment and belief in you, but if given no other option you will find something else because you need to be compensated for the job you’re doing and be thinking about your career longterm.