Hi Birroc,
Rentokil have visited 5 times now with the traps still down. There are no signs of rats now since mid January. Under their advice I reluctantly left the access hole from my garden to the sewer open. The poison they put down was in a gel base block which I guess meant that the rats had to eat it straight away as oppossed to bringing it back to their nests. After no signs of rats in three weeks I sealed up the hole with a mixture of glass, chicken wire and cement. I also jammed a metal cake tin down the sewer inspecton hole without blocking the actual drains and put poison on top and below this. If they come back I may sell up!!. I'm hoping that these rats are now dead and that any new ones will not realise this route ever existed.
I have no association with Rentokil.