My Hayter 10/30 ride on mower won't start, any ideas plz?

My Hayter 10/30 ride on lawn mower has never been a problem to start. I have recently needed to use a power pack to jump start it, as it has not been turning over without. I assumed it was due to the battery needing replacing. A couple of weeks ago the mower wouldn't turn over using the power pack, it just clicked. I replaced the battery, no different. Checked the switch under the seat, that ok. Short cut across the connectors to the starter motor and bingo! Took connectors apart and cleaned them replaced them and bingo! Put the mower away, as it was dark by now, went to use it yesterday and back to square one. If you short across the connectors it turns the engine over but it will not start nor sounds like it's firing. Not sure if the 2 problems are related but it is really annoying an frustrating, anyone else had this nightmare or can help with any ideas please? Cheers. Andy