My last tankfull lasted for 123 miles, which I thought was pretty bad and I've just run out of juice after 103 miles!!. If I'm right then 13 litres in 103 miles = 36 mpg. My question is why such poor consumption and are the heavy plumes of white/grey smoke on warming up some clue. I've checked the plug and it does not appear to be running rich. The only thing is that at times (with the choke shut off) it idles at 3000 rpm untill I bleep the throttle.

Any ideas.

White / grey indicates overrich, no wear inside to be afraid of (yet).

Most likely causes in descending order of probability (IMHO, of course):
1. Choke plunger sticks, blipping throttle makes it close or something. Closed choke does not mean necessarily that it isn't operating. Push back choke plunger at carburettor. If that does not help, remove choke plunger and check for smooth operation. Alos, check cable.
2. Float problem. Dirt / worn needle / needs adjustment.
3. Second stage throttle slide sticks in open position. Clean, preferably replace.
4. (very) Dirty air filter. Clean.

2 and 3 need carburettor dismantling. First check 1 and 4, takes 10 minutes.

Excessively overrich mixtures wash away the lube oil film from the cilinder wall and so may cause damage when not cured, just to make you sleep well tonight.

Succes!
Auke