Batteries just turned up, Energizer Lithium 2025, put one in worked straight away, no re-pairing required despite being powerless for over 24hrs!
Odd that the battery I had in the drawer, still sealed etc wasn't up to it, wasn't even out of date or anything but clearly was dead. That was an Energizer (non lithium) one too!!
Least they'll be a nice post here for people to search in the future for assistance, thanks to all that replied :-)