I say this with a hint of hesitation because I don’t want to curse myself, but, issue seems resolved.

I disconnected the battery having already disconnected the VSS and since that - the idle rev has now stopped!

I still have the heat coming through vents but this is likely the HCV and once the part arrives and is fitted that should be the answer to that aspect. But so far - so good.

thanks to GVK for the advice!