Yes, I agree the boiler size is oversized for two people, however, having done the heat loss calculations we are talking about 26kW on heating alone, (have consulted with several companies). This seems to come down the the fact that the house is big (9 rooms (exclusing baths / hallways and cellar - will be used, and therefore heated) and all are large with high ceilings (smallest is about 3m*3m*2.5m). The house is also old (single glazed, solid brick walls - so no potential for insulation there).
With regards to the size of the hot water tank, although currently only two people live there, it is definitely a familiy house, and although I don't yet have a family I appreciate that accidents can happen! I also feel that while we're putting in a new system, we should make it adequate for the house, rather than for us, as previous history would show that we would be moving on at some stage, and wanting to sell the house (appreciate though that the size of the DHW cylinder isn't most people's first consideration!)
I was therefore considering the 40kW, because allowing 4kW for hot water seems rather measly (loads of people have recommended worcester boilers, and I think the next size down is the 30kW). however, I'm asking for advice, from people who I suspect are mostly highly competent plumbers, because I don't know (and I appreciate I don't have the experience) - so thanks ever so much for your response as I very much value the advice I receive. If you could have a read over this and see if the calculations I've had done still sound excessive, I would greatly appreciate it - (Don't want to spend money unnecessarily!)