£800 for a survey sounds like an awful lot. You might actually be able to get the Freehold itself for just a bit more. As far as I know the rules on selling freehold are not set in stone. The amount a Freeholder can pay for freehold is not necessarily based on anything specific, for instance the remaining years on your lease. Don't quote me on that though. What I have heard though is that it really is a bit of a lottery. You simply put in an offer that you think you can afford and the freeholder accepts or rejects it. 

I would also say that 25k for the conversion itself is a bit steep when you consider that alot of the work can be done yourself if you're a little bit 'handy'. Laying insulation is unpleasant if you use Rockwool / Glassfibre but it needn't be if you use a cheap paper- overall or work cloths from Oxfam and a basic face-mask. Alternatively you can use Sheepswool or expanded polystyrene. I know it sounds crazy but wool is totally appropriate; it's relatively local, helps the struggling UK sheepfarming economy and as long as it has been treated with Boron Salts it'll not attract moths. Here are some more good options…

http://www.which.co.uk/advice/…../index.jsp

Laying a floor is simply a matter of buying a cordless screwdriver (anything from 30 to 100 quid), a rip-saw ( http://en.wikipedia.org/wiki/Hand_saw ), some screws and getting chipboard sheets delivered from your local B&Q. You don't even need to pre-drill the holes with Chipboard and if you're not confident with a 'Cordless' then use 2.5 inch nails and skew/tosh them (nail them in less than perpendicular to the board's surface) so they won't work their way out—as long as the all fit tightly.

If you do that then you will have saved yourself a considerable amount and the builder can come in and do the rest; like fitting lights, cupboards parallel to the eaves ( where you wouldn't even be able to crawl ), putting in windows and laying carpet. If you already have floor insulation up there then you just need to check if it is still fluffy and maintaining it's required thickness. Over time they flatten a little particularly if there is a lot of dust… from previous roof work or simple climate related expansion and contraction. Besides, we have more demanding building regulations now than we did 20-30 years ago.The Loft insulation underneath the roof-proper is something which will be more difficult mainly because fixing boards over the roof joists can be very strenuous, especially if you're working alone. You will probably want to leave this to a professional.

To be honest, the most disruptive piece of construction work you may need to have done is the addition of a rolled steel joist (RSJ or steel i-beam in lay-terms) to take the load of a newly habitable space in the roof. And this is why you really do need a surveyor or qualified architect (RIBA part 3 ARB certificated) to look at it.

Hi, firstly you have to get permission from your freeholder. This involves submitting drawings to them so they can take a look, which involves an admin fee. You then have to pay for their surveyor to come and take a look, which I think last time I asked was £800. You don't need planning permission because it's a loft conversion (under 40 sq m). Once the freeholder says yes you can then look for a builder. A neighbour had his done for approx 20k (he didn't have a dormer). Neighbours opposite paid 25k for theirs because they had a dormer and a bathroom put in. This doesn't include architects drawings which are a futher £500-800.