Search for freezer alarm; nowt Search for fridge alarm: several products intended for freezer use.
I'd given up so I had a look on Amazon. But then thought to try 'fridge' on CPC. Ended up buying three from CPC for less than two would have cost from Amazon.
Oh and I've bought two 17Ah gel batteries for Medium Bug because no-one seems to have stock of the SLA ones except Farnell and they want twenty quid over the odds. I'm hoping the gel ones will be OK.
Medium Bug by the way is a Pride Go Go Elite Traveller scooter. The batteries supplied were clearly not the best available, and they have dropped from a theoretical 17Ah/15Ah to 9Ah/5Ah in a year of light use. The other issue was the fact that the machine would stop on slight hills due to the thermal cut-out operating. It was rated at 15A. I've replaced it with a 20A one. I complained to Pride about the poor performance on hills and they just f***ed me off.