petrol lawn mower keeps cutting out on me i have petrol in it wat can i do

Get it serviced, or get a friend who's good with cars to have a look at it for you.
Another thought - the "choke" might be set wrong, and flooding the engine.

new plug or clean the old one although as Readejahn says it needs a service most likely

yes mine did this and the plug needed cleaning and the gap resetting. then it was fine.
and yes i did do it as hubby was late in from work and i'd offered to do the grass. I think he knew it wasnt a happy bunny as he smiled that all knowing gotcha smile.