Thanks for all the comments. The Byron system I have with the
extender/repeater is claimed to have a max range of 200m. This is obviously
for those of us who live in a field. If you live in a house with brick walls
then this reduces the range enormously. The distance from my doorbell to the
workshop is 30m tops. With all new batteries and the extender positioned
just right then it does work but it is at the limit of it's range and after
a couple of months when the batteries start losing their charge then it
becomes very intermittent. Trouble is you never really know it isn't working
until delivery men start dropping cards through your letterbox.. I've asked
the question about using a second extender to Byron's tech support so we'll
see what they say. Maybe the simplest option is just to get some decent
rechargeable batteries and just keep swapping them over say once a month.

John