You haven't said what motor it is, but presumably it will be a good old Briggs & Stratton.
Knowing Hayter as being one of the better garden lawn mower manufacturers if it is a B&S it will be to an industrial specification and not like some of the engines they also make for the throw away market.

My suggestion is to first of all, drain off the petrol tank and refill it with VERY fresh clean petrol.
Second, buy yourself a new spark plug.
Thirdly, take the air cleaner off and have a good look at it. Is it a sponge filter and does it look to be well and truly full of muck? If it is, take a small bowl, put a small amount of petrol in it and wash the air cleaner out, then do it again and then dry it.

Put the whole lot back together and your engine should now start and run sweetly.