Greedy and ladyalex have said just what I was going to say - you need the Occy health view on this. Anyone who's been on long term sick in the NHS should be referred to Occ Health anyway, to make sure they are fit to return - and if your manager hasn't seen fit to do it, you can refer yourself. As others say, if you make your own referral OH don't sent a full report to the manager (which they do if the manager makes the referral). Anyone I know who has been long term sick has nearly always had a phased return, so they don't find themselves back where they started by going in too deep, too soon.
Good luck with it, tinks - I know how it is, as do many of us on here - hold on tight, let us know how you get on.