Hi all, thanks for all the ideas and suggestions.
Ill try and answer them all but its a bit tricky to 'quote' them

Engine is definitely a 4 stroke. It has seperate oil and petrol
reservoirs, and I havent mixed oil/petrol like I understand you do with
2-strokes.
The oil being used is B and Q 4 stroke lawnmower oil.

Im holding down the dead mans handle constantly while mowing, its not
stopping as a result of me leaving go. Admittedly it took me a few goes
to understand this though hehe

I dont think it has a choke as such. Does that make any sense? Ill
double check this.
The pushbar has a control lever on it, saying 'slow, fast, start'.
Maybe the 'start' setting is equivalent to the choke ?
I start the mower and then move this lever to fast, mow for a bit and
it generally cuts out after 30 seconds of this. There is no backfiring
or spluttering, it simply starts slowing down and then stops, in the
space of 4 or 5 seconds.

Does it labour as if under a lot of laod whic eventuially overcomes the power of the
engine?
I guess it could be described as this yes. The grass isnt very long,
but theres bits of uncollected grass lying about from when my electric
mower packed in. It doesnt look as if it should cause an engine much
difficulty though.

Ive read the instruction manual and and following them pretty much to
the letter.

I bought it from B and Q, and given the choice between a mountfield and
this one, I chose this one because it could collapse a bit smaller (Im
limited for space in my shed.) Sorry, I dont know the brand at the
moment, but Im going to be trying with it again today so Ill find out.

Likewise, I havent checked the oil using the dipstick, but I will
today. But I did check it was empty before filling it, and put 600ml in
as the manual suggested. Also put in 1 litre of unleaded petrol.

Im off give it another go, Ill come back with engine power and brand if
its still struggling.

Thanks guys
Paul