Ok, I have some good pointers here gentlemen, many thanks.
Steve, I appreciate the quite detailed remote locking programming guide, I shall attempt the manoeuvre and report back with my findings.
Ian, I suspect you are correct, a malfunctioning central drivers door motor seems to be the likely culprit here, it would explain the sluggishness and silence of the key opening the lock and why the central locking does not work.
I wanted to add that I only had one key with the car, the master which was broken and superglued back together. I have purchased two new blanks with empty innards and a second genuine Lexus 3 button key with innards to reprogram, leaving me with two working three button keys.
I see the new central locking motors are only twenty quid on eBay, seems like an easy fix if it is a hardware failure.
Bit by bit, I shall restore my beautiful Lexus to her former glory. I love these IS200's, its my third one and I much prefer them to the newer shaped models which seem to have not much of a personality.
Mine came with some (actually quite decent) third party alloys on her, so I just bought some newly refurbished genuine Lexus alloys to replace them and bring her back to where she would have been when she was new.
She has needed so far, front and rear brake pads (175 on ebay) a new passenger wing mirror and headlight (100 on ebay), two new internal ashtrays (40 on ebay), but other than that she is immaculate. Will post some pics.