flat roof replacement

Hi,

I need to replace the flat roofs on both dormers (1.75 x 5 m each) as the felt is bubbling. The decking seems in good condition (I've walked across it) but the cladding on the dormer cheeks needs replacing + new lead soakers and flashing.

I have got a quote from a roofer in the National Federation of Roofing contractors for £4,148.40 to put a warm felt roof on both dormers and £1,170.00 to put new PVC dormer cheeks, lead soakers and flashing. So £5,318.40 in total! Another roofer has quoted £2,790.00 to do all the work with kaliko (PVC membrane) so nearly half as much.

I'm keen to pay someone that will do a proper job but that's a huge difference! How much does it normally cost to have a new warm roof fitted?

Thanks,

Tom