Thanks to all for the feedback, much appreciated. I have taken the following action so far:

1. Replace the glow plug (it was black around the rim but still glowed).

2. Adjusted the tappet clearance.

3. Checked the voltage (when it was about half full) at the battery - 1.20v and at the glow plug 1.11v. Not sure if this is an expected amount of voltage drop or if I should expect less of a drop, is there a rule of thumb here?

4. Bought another battery 2400mah NICD so see if this makes a difference, maybe the current cell is a faulty as it only provide 1.2v when half full.

5. Checked that the battery stops operating above 1/4 throttle with a volt meter - all good.

6. Note that when I heard the burbling sound it was a humid day - I assume humidity makes engines run richer?

7. Am considering installing a remote glow in parallel with the on-board system to provide additional power for the start up process and allow me to charge this cell with the wing installed.

Thanks again for all the feedback, I will keep you posted on the outcomes.

All the best, Simon