Intermittent idle problem

My car is going into the garage on Tuesday for a service and new cambelt/water pump anyway but recently I have had the occasional problem with the car when starting. It tends to only happen when the car is already warm but has happened a couple of times when cold too. When I start the car it is like I had my foot on the accelerator a bit and the revs jump upto about 1200 but then drop back down to normal (about 850ish). Afterwards it can be a bit uneven and tends to drift up to about 1200 revs when idling for a couple of minutes after starting. If I then drive the car when it goes back to idle it is normally fine again.

The car dirves fine with no loss of power or funny noises (apart from it being a diesel).

This probably on happens once or twice every couple of weeks and I haven’t had any lights come on the dashboard. I’m going to ask my mechanic run a diagnostic and look at it but I wondered if anyone had any ideas as to the cause before I take it in.