If all else fails......

Yes, I know I should have dug out the instructions first, but, really, who does that?

I discovered the doorbell wasn't working when a loud thumping on my door alerted me. It's a glass door, so I was quite alerted!

I wasted a good  30 mins changing the batteries on the two theoretically wireless connected bits, fiddling about. No luck. Bought some more. Tried again. Then I gave in. The key word was synchronise. Took out all the batteries, pressed buttons as directed, done in seconds.