Hi Grumpy,
I had a quick look at the piston whilst i had the cylinder head and block off last night. My novice eyes didn't see any scoring, there was rather a lot of coke build up on the top of the piston and in the exhaust port but that was it. I've cleaned the exhaust port but not the piston head.

I had the same thought as you regarding the spark plug, i tested the spark by resting it against the cylinder head and pulling the starter rope.

I forgot to say, I had the engine running perfectly until i turned it on its side to clean off the grass from underneath to tighten the blade nutes. After this it wouldn't start. That was 2 weeks ago! Since then i'll recap what i've done;

1. Cleaned and rebuilt the carb, checking the needle and float, the fuel intake needle, diaphram, also checking the popit moves correctly when moving the throttle cable. Also i've checked there is a seal when sucking on the governing breather pipe.

2. I've replaced the spark plug and all breather and fuel pipes with new pipe.

3. I've checked fuel is getting into the float chamber and up to the injection needle.

4. I've taken off the cylinder head & block, cleaned the exhaust port, inlet port, and put back together.

Last night the starter rope broke, so i will replace that, then i will try starting it with the exhaust removed like you said.

I'm really glad of the advice, i am stuck you see, the engine seems to have spark, fuel and the air intake pipe is free and clear.

I'll let you know when i've fixed the starter rope and tried to start it with the exhaust off.

Cheers,
Trev