So we called in a pest control company to get rid of it. This consisted of a very talkative old fellow squiting poison into the nest from inside our loft. Apparently the poison they use is in tiny balls that are coated in a substance that humans can't break down but that the pheromones of wasps do break down; it then gets into the wasp through glands at the back of its head which the wasp rubs continually to make sure it has the right smell to be accepted into the nest.
We stood in the driveway chatting as the wasps spiralled down out of the sky; then we noticed a tremendous buzzing noise coming from nearby and realised that the wasps were coming out of their nest and then falling immediately down the drain pipe and collecting in big heap of dying wasps.
The man said that there would have been something like 18,000 wasps in the nest and that they look for food more than 2 hours flight away, so it would take a couple of hours or more for them all to come back to the nest and be poisoned. He said that a nest doubles in size every 7 days and it used to be very unusual to get such big nests before the end of summer (September), but the weather is so mad nowadays that the season for wasps is starting earlier and lasting longer. He showed me a ping-pong ball sized nest in the loft in which he said the queen had hibernated during the winter.