That's a helpful thought. I've looked at the aerial view on Google Maps and don't see a mast, although the photo may be out of date, so I'll look out for one next time I'm passing. However, there is a cluster of farm buildings close by, and they might well have a transformer, either one for their own use or one they've leased out land for.

 

Good thinking. Yesterday I purchased some new batteries, changed the one in the main fob, and stored the others (they are well in date) as back up, on the basis that there's no point in putting a new battery in the spare fob if it'll run down even when not in use.

 

Cold will do it, as will a flat battery, but phone masts, BT landline microwave rebro masts and (occasionally) mobile phone blockers will also cause this. You probably know that you can change the code to a number of your choice even if you don't know the present code, as long as you have a working fob.
My problem is that I can never remember how to use the code, so I carry the little card from the back of the user manual in my wallet. Old.