Hi Alex,
I had a 3 Litre 2000 model until I traded it last year, and had a similar problem with mine affecting eventually the F/N/S, R/N/S, and R/O/S locks. The problems initially started when the car just refused to lock either by fob or key in the lock. Eventually, I found the doors would lock but the rear doors would refuse to open, then would, then wouldn't. It turned out that the electrical mechanisms in all the doors (except Driver's) needed replacing. From memory these are really expensive, over a hundred a throw, and so I took pot luck from ebay and bought second-hand from scrappies. Luckily all but one worked well, and I replaced that, again with a pot luck ebay find. Thereafter it was fine for the two years I had it before replacing the car. It's a fiddly job so I got my local garage to do it, he charged an hours labour for all the doors. It might not be in anyway connected to your issue but may be worth looking at.
Mark