Reply to Evs and Irene
Central heating has done much to control death-watch. The beetles are associated with fungal growth in the timber when it is felled and cut into beams. Outdoor seasoning will firm up many timbers ready for building use, but once indoors, the constant drying effect of radiators probably alters the microhabitat in which fungus and beetles are developing. Added to this, once adult beetles emerge into the air, they rapidly dehydrate and die. The best place to find 'active' death-watch in a building is not to examine the timbers looking for beetles, but search around the carpet where it meets the skirting board for dead ones.
Reply to Nicky
The trouble with death-watch is that they DO live in inaccessible timbers, often in the very centre of structural beams and supports. This makes them difficult, or perhaps impossible to eradicate completely. The main thing is to monitor the situation year on year, and carry out the invasive building repairs only when really necessary. In the mean time, your central heating will be doing its bit to control them.