Freelander central locking fault

Morning all. I'm a new member and I have just taken ownership of an old, but well looked after '02 plate Freelander 1 TD4 Softback but I'm having a problem with the central locking system and any thought/advice/suggestions would be welcome.
The key fob remotely locks all three doors and sets the alarm. On unlocking, the fob releases the front nearside door and the tailgate, but the front offside door will not unlock. However, it does unlock on insertion of the key into the exterior keyhole. I have found an unconnected multiplug under the drivers side seat with a black wire and a red(ish) & white wire, but no obvious male socket to fit into it. The only other non-working electrical circuit is the steering wheel mounted, volume and mode control for the audio system. Might these "discoveries" be significant?

Don't worry about the plug under the drivers seat....we all have one - It's sounds like you need a new solenoid for the door lock

Hiya Tiny. Thanks for your suggestion - next question is where do I find the afore mentioned solenoid(s)? Does one solenoid operate all the locks or is there one for each lock and they trigger in parallel? Can I assume that if it (or they) doesn't 'click' when the fob is pressed, it's duff? If or when the solenoid is changed, will the fob require reprogramming or replacing? Sorry to be a pain but I want to learn!

The door lock latches are common . so just take one out and put new in . They are all controlled by the CCU on rear of fuse box. IF you had to change that then the fobs would need re programming , otherwise no. The door latches contain two motors , one does normal locking and the other does superlocking . HTSH

Thanks for the advice and now I feel reassured to get the spanners out!
Maybe from what I've already read, it might also be useful to stock up on Savlon and plasters first !