Orangery costs and payment schedule (North West UK)

Hi We have had a number of quotes for an orangery to be added to our detached house. The specs are as follows: Size - Depth 4.8m and Width 6.8m so planning permission required Specs are plastered 100% throughout Ultraframe roof with glazing (self clean and solar reflective) side walls are 3/4 with windows dwarf to front with 4 X 700mm windows plus french doors 1200mm plus side windows Cost we've been quoted is around £33k or around £1100 per sq m. Cheapest quote is £28k Does this sound reasonable? Also payment schedule has been suggested as: 10% deposit 10% on planning application granted 40% when all walls in and ground works done 30% when windows / doors / roof are delivered (tempted to push for installed here) 10% complete I think 20% at the point planning is granted is too much and will suggest to move the 10% to when the windows, doors and roof are in (i.e. this payment will be 40% instead of 30%). Any advice greatly appreciated given the rather large spend!