As a father of three sons, all of whom seem to have survived the rigours of the nanny state, I'm extremely sceptical of places such as the one you vistited. Their main aim is to liberate you from your hard earned often by selling you a load of stuff you don't need e.g. steriliser, if you haven't already been conned already on that one. We had all sorts of fun with varous cars we were driving at the time: MR2, Audi 80 and the one that got a completely blank look, a Caterham. The simple truth is that car seats both front and rear are designed for lardy great adult arses and no child seat in the world is a perfect fit no matter what their list says. Common sense is your best guide. The object is for the child seat to sit down reasonably snuggly and not move about unduly under heavy braking. If you can achieve that then little monster will be as safe as you can reasonably achieve. Try borrowing off anyone you can (the dreaded NCT mums) and test them.
GTV and Racing Driver