Ok well here’s what happened:
Setting the alarm to leave, had to go in and out about three times (and set and unset) in quick succession due to forgetting things. The last time I went in it wouldn’t un-set. Typed in the code, nothing.
Had to open it, unplug the battery and turn off the power to get it to shut up, but it re-armed every time I reconnected it. I thought it might be the battery in the keypad, but changing it didn’t help.
It may be that the keypad is no longer linked to the unit, not sure. Might need to be re-coded or something but that takes specialist kit presumably.
It was a monitored alarm installed by St John Security of Bristol in 2007, if that helps. No longer supported by them though, since we cancelled the monitoring contract due to funds.
I wouldn’t mind the potential to have it monitored later, but that would preclude DIY I think. does £500 seem a fair quote for a monitored installation?