Key remote/central locking possessed

[L4OMEO]

Hi all

The lock/unlock button on my remote key (the black flip-out style) stopped operating a few months back. I tracked this down to a dry solder joint on the PCB, & fortunately I work at a company where technicians solder very small components under microscopes so I had this successfully resoldered and it's been working fine. Oversaw the job and was quite happy it was done well.

Over the past few weeks though, whenever I come within range of my 156 with key in hand the central locking starts randomly and continuously unlocking and relocking of its own accord. Presuming the car's not haunted (it's possible) my first suspicion is that this relates to the earlier repair. If so, and if this is going to give ongoing issues, then frankly I'd prefer just to replace the key and keep the dodgy one as a spare. BUT, I want to be sure this IS the key at fault, so has anyone else had this problem? Could this be a faut with the central locking/receiver rather than the key? Also, where is the best place to buy a key? I know it will need to be coded by the stealer ... sorry, dealer, but is there a cheaper place to purchase the key itself (and what am I up for)?

Cheers!
Rory