I've got an identical problem just now Mike.
Before, when hot starting the RPM would fall back to just over 1000 (as it still does from cold) and then settle to just under 1000 a second or two later. Now the revs just drop straight down to below idle and it just stalls.
Before it got really bad it was dropping down to below idle after starting and then hunting a few hundred revs above and below idle, but usually settling down to idle after hunting for a handful of cycles.
Also, before, if the clutch was depressed and the car coasting (eg at a junction) then the revs would hold up to above idle but now it goes to idle or occassionally below and has stalled twice, but is generally OK, although the power steering stiffens very slightly as a consequence.
As for the cause, after reading past posts on here and speaking to a few people, it could be the potentiometer/throttle position sensor which isn't a huge job, but if it isn't that then there are any number of things it could be.
It's manageable at the moment - on starting I give it a bit of throttle and then let the revs drop slowly to below idle and it generally settles at idle after that, but my MOT is due soon so I'll have to get it sorted.
No doubt some of the more knowledgable posters will be able to shed some light, but if you get it sorted out before me then please post on here, and I will do likewise!!
Stu