The other night I found one of the keys wasn't working, so I tried the battery from the other one and then neither of them worked; brilliant.
So I googled & found a reset procedure; open with one key, put ignition on, close door, lock with other key, press and hold both buttons until it does one beep of the horn.
Well that kind of worked; the key from the second half of that now operates the central locking but then I put it in the ignition & the alarm goes off. Turn it more and it starts the car but the alarm is still going. Lock it on the central locking and the lights are still flashing. Unlock with the central locking, ignition on, alarm sounds.
Have tried various versions of the reset procedure including another I found on here but the alarms still goes off when the ignition is put on.

Have disconnected the battery now as it's outside my neighbours bedroom!
Interestingly we had a wet passenger side carpet until about a month ago but stopped using the rear wash-wipe until we take her for a service so I doubt if that's the problem.
Thanks for reading/helping!