Am I right in presuming that it takes a good few hundred miles for things to correct themselves when you change to a higher octane fuel?
Also what are the recommended plugs? I will have a look at mine tomorrow.

Just solved similar after two years of this intermittently.
The rubber connecting airbox to the link pipe that leads to throttle body had split under the hose clamp so was hidden during casual inspection. The throttle body end of the same pipe had the same issue. The consequent airleak was intermittent but would cause hesitation at low revs. gaffer taped it this afternoon whilst awaiting new rubbers and problem is gone. At last.

HTH

Bill

I don't suppose you could get a picture could you.

I haven't done anything to the car because just not had any spare money lately but I have been using a higher octane petrol.

The car is very brisk when cold but the problem rears it's head when warm now.