Vauxhall meriva 1.7 CDTI - Starting, not revving - rincewind

I have a 52 plate Meriva. Runs perfectly well when it does, but has a very intermittant problem.

After a run, and being left for an hour or so, It will start, somewhat reluctantly, and run OK, but will not rev at all. When left for a few more hours it behaves perfectly OK. A bit embarrassing when it does so for the RAC man !

Any ideas please, or should I beat it over the bonnet with a few stout birch branches ?