Apparently as this tree is within 2m of where the foundations are, the foundations need to be a little bit deeper... 1.5m altogether (as opposed to the 80cm - 1m which had already been dug; for comparison, the foundations of the existing house are somewhere around 60cm deep)
So, a bit more digging to be done there, and yet another skip needed; I'd arranged for the skip to be picked up tomorrow so that the concrete lorry could get to the driveway, but have now swapped that for an exchange.
Still, looking on the bright side, if an earthquake ever hits the Epsom area, our house is unlikely to go anywhere. Well, the extension bit won't, in any case.