Thanks for the info.
Monkey, yes I had wondered about all the background noise and as there is a partial steel roof fitted (loud in the rain) so would have to be careful where setting up the monitor or set the receiver to such a low volume I would only hear when significant damage was being caused. This might be a little late as I want an early warning system to back up a mains alarm already fitted.
Pies, that system looks like it would do the early warning job and the comments about battery life - a possible weakness - all look positive on the reviews. Bonus.
Pym, I intend to do the same on the side door (notices) which is not visible from the road or by neighbours.
Now just need to leave my Michael Myers Halloween suit handy to greet anyone....