On reflection, I agree. I'm going to use 300mm uPVC cladding, which gives the same effect as the 100mm t&g I originally considered, but works out cheaper and will (hopefuly) go up quicker.
50mm on the walls, push-fit between tiling battens screwed to the concrete sectional sides and then clad with OSB, 25mm on the ceiling, supported by tiling battens screwed to the original cross-member joists. With two secondhand (and extremely cheap) uPVC windows and an ex-display Compton garage door, I'm quite pleased with the results so far.Bert