I've just spoken to Dave at Fiorano's who worked for Meridien for years, and to disconnect was exactly his suggestion, and apparently what Meridien's used to do as standard procedure when people had the same problem. The car's going back to him in a week or so (for a rear arm that was on back order and missed being done at the service) so I'll find out why they always took that route rather than adjust.
Holding my finger on the remote til the beep will have to do for now. Apparently the other night my Son had gone out to the car to fetch something from the boot and armed it normally, so that explains that one.
Huge thanks for all your help and pointers on this