Further forays into the house by the mice were spotted today by both of us. We've figured out that they are coming from a gap at the back of the fireplace in the study, so Jase has plugged it with copious quantities of tin foil (we figured that this material would be both fire proof and chew resistant).
We will need to see if this makes any difference, but obviously there may be other access points into the house. However, of the four mice who have ventured into the house, all either emerged from there or ran to there to escape, so hopefully this will be a temporary stop gap.
Once in the house they head straight for the sofas or the kitchen. Much like the humans then!