My 2009 rrs recently had problems with the transmission. It starts and drives ok until it warms up and then it stays in 3rd and displays F on the dash display. If I stop and turn it on again it's ok for a while and then the problem reappears! Any suggestions please.

Standard answer at this stage - you need to get the error codes read. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

I have the Gap IID tool and none of the faults are powertrain faults (including transmission) see photos showing the faults

I had a problem with my transmission that only occurred after warm-up.
As you have a IID, try using the search function to search for 'temperature' and select all of them under 'live values'. Then go for a drive long enough to warm up whilst recording all the temperature values. Send the recording of the values to yourself via email then e axamine the temperature vs. time graphs for all the temperatures. You need to use a PC to look at the traces as the display on the phone is too small to see the necessary detail.

Look to see if anything off about the temp values coincides with the onset of the fault.

I used this method to identify a faulty temp sensor in the mechatronic that was causing the transmission fault  2008 MY Java Black TDV8 HSE