I wrap my 4x2 studwork base plate in a 300mm DPC and screw through it to anchor to the concrete, (won't this pierce the DPC?)
Yes, but don't worry about that. It's accepted practice to fix through a DPM - it still protects 99% of the material, which should in any case be treated timber, otherwise how do you get a structural fix?
then I fill the internal space with a DPC sheet lapped up the stud work to above ground floor level.
I'm assuming that I will have to make my internal walls longer to accommodate the extra (below floor) construction
I think your sequencing is a bit out, here. I'd think in terms of building the wall frames and applying a moisture barrier on the cold side of the insulation (i.e. the outside of the framing), which should be a vapour permeable breather membrane such as Tyvek or roofing membrane, cladding the outside (consider battening off the framework to get a ventilation space, too) and sealing it to get it relatively weather tight, then getting the roof frame on, applying a moisture barrier to that, adding cladding that and felting it (or whatever weatherproofing system you require, such as fibreglass or Sarnafil, etc) so that you have a relatively weatherproof structure to work in. Next I'd put in window frames, door frames, glazing and doors. This gets you the main structure.
One minor wrinkle - consider adding in a pattress on the inside of the wall frames where the skirting will go. Rips of 150 x 12mm OSB will do the job. It will make the job of fixing the skirting later on far, far easier
At that stage I'd run any wiring and other services I require, then insulating the gaps between the wall studs (for preference I'd use mineral wool batting or possibly sheep's wool - better for soundproofing), fix a vapour control layer on the warm side of the insulation (this can be 1000 or 1200 gauge Visqueen - same stuff as you'll use on the floor - or a purpose made VCL) and finally cladding the inside with plasterboard/timber, etc. Once the walls are in the floor membrane and PUR insulation can goes down in a relatively dry environment. Try doing it before the structure is up and all the tracking about with wall insulation, membranes, wiring, ladders (the roof, remember?), etc is likely to shred it and make it useless, hence avoiding doing it in one hit. The PUR I'm suggesting is fairly rigid (hell, we lay it onto concrete slabs and pour 50mm cement screed in over the top of it - and it supports commercial kitchen equipment) and has good insulation properties, although not as good as blown PIR insulation like Kingspan or Cellotex
BTW I'm assuming a lot about the structure of the wall there, but it's something like what I'd expect to see on a build. That said I also don't know if you are going for a warm roof or a cold roof, hence the lack of detail about the roof (you obviously need to consider ventilation, etc)
If anyone has a cheaper way of doing this I'd be interested to know about it
