Sounds like too much air getting in somewhere. How did the problem first show itself and had you changed anything before it started?

<edit to add> At the back end of the inlet manifold there's a rubber cap on an unused* hose barb sticking out of the back of no.4 inlet runner. Check that hasn't fallen off or split. (It never actually is that but the one time nobody asks it's bound to turn out to be that. )

*Unless it's an automatic.

 

Spray the intake pipework with carb cleaner - if the revs rise you have found the leak.

 

I had this problem and changed the iscv for a good one which turned out wasn't good. 2nd good one I tried fixed it after a week of fiddling about and changing everything else connected and lots of swearing every day. The idle would start around 2000rpm cold and drop to 1500rpm once warmed up.

Have you also checked that the coolant is circulating through the air valve hoses? I had one were the pipework was blocked where the small hoses for AV exit near the inlet manifold. Years of running water instead of coolant had caused it to clog up with thick rusty sh!t.

The ecu engine temperature sensor at the back of the engine can also cause high idle, but less likely to fail. It needs the coil pack removed to access. Not as tight to get to as it seems. If you can get a deepreach socket over it its easier than getting a spanner in there. Tapered thread, so once it's turned a little, its easy to take out the rest of way out by hand.

Unlikely to be the cause, but have you checked base idle speed. Check Google and on here for a guide. Theres plenty of them. Basically Short ten and gnd at diagnostic box once warmed up. If idle speed changes drops of raises you adjust it via screw by throttle body. If something else is causing fault adjusting this will accomplish nothing, but still something else to check.

Other than that, breadbin is right. Check all the intake pipework again for air getting in. Be sure its all seated right at the joins. Even a little air getting in will cause higher idle.

Had you done any work to the car before idle changed, or did it just happen out of the blue?

 

Thanks for your thoughts guys really helpful.
The car was sat up for quite a long time but I don't remember a specific act that caused it to start.
It only did about 80 miles last year so I troubleshooting the remaining niggles.