Thanks for the advice. I've planned it to specification and at the moment its:

1. 150mm ceiling joists with Celotex 120mm between the joists and 25mm beneath plus my plasterboard etc. However, this means the ventilation gap at the front will be 30mm instead of the regulation 50mm (increasing backwards as the firrings rise) U-value 0.2 W/m2k

2.Sticking totally to the regs it's 100mm celotex with 35mm below u-value 0.2W/m2k which gives me a clear minimum gap of 50mm above the insulation

Somebody over on ebuild pointed out that loft insulation must meet minumum energy efficiency values as set out in the approved document with the proviso that it is techincally or functionally feasible. If not then best standard which can be achieved within a simple payback of no greater than 15 years.

http://www.planningportal.gov.uk/eng...315785835.html

I'd like to keep headroom to a minimum of 6'10" really as anything below this seems a bit low so maybe i'll get away with it. I think i'll check out this thinsulate stuff and then sent off to BC and see if i can negotiate on the headroom.