Idle getting faster

[Justheaven]

We have a 2005 mk6 LWB diesel.
We bought it it as a minibus and have converted it into a camper.
On first starting, idle ( tick over) is at about 800rpm, after about 20mins it's up to 1200, a bit later it's just under 2000rpm.
Turning the engine off and restarting goes back to 800rpm.
We have been told that the "throttle position sensor" is at fault and needs to be replaced. I have searched on-line but can't find the suitable part.
Can anyone help?
Thanks
Steve

[Altransit]

The part number is printed on it, just remove the pedal and have a look, it's only one bolt to remove if I remember correctly
Search the part number with Google