I am getting a bit fed up with sitting around car parks waiting for an indetermined amount of time before switching off the engine and then worrying all day as to whether I had left it running long enough.
What's the general rule for turbo cool off time? And what will happen if I forget to go through the routine?
I'm toying with a turbo timer but is it a pain to fit? Can it be spliced inti the Sigma M30 alarm so I can leave the car running with the alarm activated and then expect the immobiliser to cut in after the TT has done it's stuff. Am I asking far too much?

 

No, you are not asking for too much.

You can get a turbo timer to work with an alarm system. You just need to delay the fuel and ignition cut off in the alarm by, say, 2 minutes.

A turbo timer generally runs between 30 s and 2 mins.

If you shut off the turbo immediately after having been on boost, oil will clug up in the turbo, reducing its performance and eventually cause failure.

A good way around it if you do not have a turbo timer, is to drive off boost for the last 4 to 5 miles of your trip. (I personally find that hard to do! )