How can I fix my Mountfield Empress 16?

My Mountfield mower will start, eventually after a lot of effort, but it cuts out after less than a minute or so.

The engine noise is erratic, not the full-blown roar that it usually has. It has plenty of petrol in it.

I'm not that technical so if there any simple, obvious (to anyone but me) things that I should be checking I'd be grateful for any advice.

I'm not planning to strip down the engine, or replace any but the most simple of parts, so if anything I've said above sounds like its a major job then that would be worth knowing too.

Thanks