In reply to Jamie Wakeham:
Thanks for replies, all - incredibly useful. To try to answer a few questions:
The house feels cold in the winter, especially the ground floor, and hot in the summer, especially on the top floor. I had hoped that 6kW of solar panels would mitigate the latter (thinking that if I'm diverting 6kW of solar energy away, that's got to keep things a little cooler), but I'm not really noticing much of a difference!
It is dry as a bone, though. It's in a depression and very well sheltered from pretty much every direction. The render seems in good condition and I have to roof looked at when the PV went up so I'm reasonably confident there's no water ingress to the cavity. The most used shower is on the middle floor - currently no fan to vent it but that's coming soon.
The cavities are definitely empty; he drilled and checked with an endoscope. It's common knowledge that nothing on this development got cavity insulation when built, so I wasn;t surprised, despite the age of the house.
I do want to use the loft as storage space - he started by quoting for just cross-laying quilt insulation, but I need that area boarded. No idea what type of board he's specificying so will go check that. I know I could DIY this (did in the last place, but I knew I was leaving soon so wasn't taking quite so much care) - I'd rather pay to have someone do it right. And be available to complain to if it goes wrong!
It looks like the grants are intended for people in receipt of benefits, or in deprived areas, which is fair enough.
Toccata - have I got me a cross-forum stalker..? I'm fairly sure I've not asked about that here! But no - just had a chat with Celotex about insulating that space, so should be going ahead soon.