Did you try sealing the base of the aerial?
Hi Val, yes I did, and thanks, but theres been further interesting developments.
The car had been OK-ish for the last few weeks, even after sealing the aerial hole: sometimes I'd have to lock/unlock it a few times before the immobiliser would disengage, but last night, I had to call the AA out as it had become worse.
I had found over the course of the last few weeks that locking/unlocking wasn't neccessary, I could just remove the key from the ignition, put it in again, and sometimes it would work. Well, I was trying that last night, and suddenly the immbiliser LED stayed on for good, instead of flashing quickly or lighting up for only a few seconds. The "STOP" LED to the left was also lit, and not going out.
The starter motor would turn, but the engine itself wouldn't even attempt to fire. Also, upon removing the key the remote central locking wasn't working at all.
The AA man made lots of noises about "nine times out of ten Renaults with immobiliser problems end up getting towed", but eventually he found something which helped. He removed the fuse marked "MULTI TEMPO" and replaced it after a few seconds, and after that the whole immobiliser problem has been OK - but with only the old key which originally went wrong at the start of all these problems ages ago. Confused?: you're not the only one.
He has put me onto a local garage who he reckons is a bit of a whizz with alarms and immobilisers of all makes, so I'm going to take it there for a once over (cheaper than a Renault dealership), and I'll post any findings here.
Cheers for the help.