(all I actually want to do is buy a new spark plug!)
I started by looking around my own Hayter mower's engine. Brigg & Stratton of some kind, lots of nice numbers stamped in. Took a photo, went to the B&S web site.
My numbers don't match anything there.
Though - a hah. I'll look up the mower, and see what engine Hayter fitted. Looked up my mower's numbers.
No direct match.
Can anyone either (short term) tell me what spark plug I need (long term) tell me how to navigate the Hayter and B&S part numbers?