I've checked the air filter and given it a clean, although it was already quite clean.
I've checked the petrol
Next thing to check is the oil.
One thing this page says at the bottom is to tilt the lawnmower on it's right side (away from the air filter) as this takes any air bubbles out of the system, worth trying.
Found this manual on a different model, will need to check what sort of oil it takes though.
I've seen a few videos which suggest it is the Carburettor, something clogging it up. Probably needs the air filter taking off first then the carb. Need to get some carb cleaner though.