Bob Hobden wrote:
Sounds like fuel starvation caused by dirty carb, dirty petrol filter,
dirty petrol tank, carb float not floating any more but stuck.

Took out the spark plug and it was indeed covered in soot, and the air
filter was filthy too. Cleaned both of those as best I could and replaced,
didn't have much difference. I'll see if I can replace the fuel - just need
to find a funnel. I discovered it was four stroke (it even says on top,
d'oh) and there was a small oil filler cap hidden in the muck - seemed to
have oil in it.

The engine on top should have a name on it, normally Briggs & Stratton (4
stroke) or Tecumseh (either) or if it's a more expensive mower a Honda (4
stroke). If it doesn't say B & S or Honda it's more than likely a Tecumseh
probably with another name on it though.
Take a look at the following and see if you can see your engine, then look
for instructions and troubleshooting, you may even be able to download a
manual.

Tecumseh rings a bell - I'd have spotted if it was the other two. I'll have
a look on the site to see if I can find any instructions.

My guess is it just needs a service, if it's a 4 stroke then carb' cleaned
out, and if dirty then petrol tank cleaned out *, then check/change... new
plug, new oil, air filter, if it's a 2 stroke then everything but the oil.
* look also for a petrol filter, it may be blocked with crud too.

Are such things easily accessible (have found the oil, the petrol, the air
filter and the spark plug orifices) or is it usually a 'unscrew every bolt
you can find' job?

Thanks, I'm beginning to understand how the thing works now

Theo