Thanks for the advice. I have had a really good listen with the A/C on and off, when the A/C switches on I hear the click but the engine behaviour is exactly the same.
It is beautiful to drive and I use it every day on some fairly long journeys so a trip to Zed World sometime would be fine.
I have reset the ECU a couple of times over the last few weeks, but not since I adjusted the TPS and tightened up the loose coil pack. I will give it another go later tonight, it may help, thanks.
If there is no change, I think I will check the idle control valve as recommended above.