If you want to stick with medical, I would maybe apply to your local hospitals bank service and do administration work. You will build abit of a rapport there and gain a good NHS reference aswell as great NHS experience. Bank positions are zero hours and ad hoc however you are not limited in the number of hospitals where you can go as a bank worker and it is entirely up to you when you want to work. I have heard of people staying on assignments for months; you are also not limited to specific departments so you could maybe take an assignment when one comes up doing audio transcription and build a mix of skills. if it is medical writing you would like to progress with, then you will need great transcribing skills anyway and working in administration will assist with that.
I have previously seen jobs for trainee clinical coding aswell, although of course it is not entirely in the direction you want to go in, but again it will give you NHS experience, a decent starting wage, specialism and more. I would take the chance to find a job you like the look at and look at the job description to see exactly what experience they want and their desirable and essential criteria. You need to satisfy all essential and at least some desirable to be in with a great chance of securing a job and then of course your applications and interviews need to be very polished and good.
As the previous poster says, please do not do a masters until you are completely sure of what you want to do. The expense is huge and it again, does not guarantee you a job. I mean I am exploring some options; I love the look of a museum studies course however I am not particularly sure whether it is worth it when jobs are so competitive. I'm applying for a couple of casual museum positions that last temporarily and that don't get in the way of my main full time administration job that I will be starting soon.
Also why do you consider yourself overqualified for entry level jobs? What experience do you have that can satisfy a higher pay grade; usually jobs above entry level constitute to at least a couple of years experience as if you think about the calibre of candidates that will be applying to them, you just will not have the experience required.