Not sure if you can do this where you are but, few years back we had the same thing, leaking flat roofs. Did a bit of research and it turns out we could apply for a grant from the council. With that you could decide either to be means tested or just go for a 50% grant!
Well, I thought to meself, why the hell not, after all I have been paying them council tax all those years might as well get something back. Went the 50% route, had to give them all my financial details well, just the same as if being means tested, apparently, it's just a formality when they see the 50% box ticked they just approve the best quote. All has to be to the latest building regs and all that but that house got a 25 year guarantee on the work done, nice new fibreglass roofs...
All part of the plan to get it done up and sold!
Have a gander at what the council might be able to provide you with. Oh yeah, and it was private owned house, not council or even ex council stock.
Might help a bit.
Jon