it's timber frame with plasterboard, 100mm celotex, exterior-grade ply which is rendered, built 2005. like a caravan reallly - too hot in summer, too cold in the winter winds - dormer rather than slopey bit seems to be the problem.

Q1: is this compliant with regs? seems a bit rubbish
Q2: How long might it last (assuming well-built)? 5 years?

Loft conversions are often built lying to building control. It is often for a peson to claim that a room will only be used as a office space or storage and then they put a bed up there and claim it is a bedroom. I don't know what you are using the room for.

The loft conversion will have been made to the building regs of when it was applied for. This could be a few years before the work was actually carried out. Most of the time a building control officer will have come to inspect the work after to sign the job off.

Sounds like it may have been a cheap job. Is there a lot of drafts up there? If so then it will be the drafts thats making it cold in the winter. I would suggest some draft excluder's around any entrances to the loft if there are.

As to how long it will last you will never know. If it's a shoddy job and you want to use the loft for whatever, then its going to last until you replace it to use the loft properly.

Lofts always get hot in the summer. Tiles and such generally absorb the heat then it passes into the loft. So to keep it cool you can either use reflective insulation in the roof or just open a window in the summer.