Thanks for all the advice - thanks @Tangoup51 and @vickster in particular. In the end my doubts about button cells meant that I passed on the alarm lock (for now - may go that way later) and I have other alarm things I could secrete in my tourer's panniers. So since delivery is free at the moment, and returns also, I have gone for the Mountain Warehouse lock as suggested by vickster. Some quick measuring suggests that it might fit very snugly round the 26inch wheel Exped bike's front wheel and a bit of the frame, so I could use it like I use my light Decathlon cheapo special on my London town fast bike. Have a vague memory that I have seen this lock in Mountain Warehouse shops and been none too impressed with it but it may possibly be ideal for my intended use. Have no fears folks - it won't be used as a primary lock in London. Will try to report back. Thanks again.