In what might be called a providential happening, there is a hard rubbish collection in this area at the moment, it's been some 10 years since we've had one.
As I drove to work this morning a sign on a piece of junk caught my eye: it read Briggs and Stratton!
I spoke to the owner who was still in the act of dismantling a Deutscher ride-on so the collectors would take it - and to my surprise found it did indeed have a Briggs engine, an 11 Hp model.
I brought it home, cleaned and re-gapped the plug, added some fresh fuel, and after a few pulls on the starter away it went and ran quite sweetly.
So the plan will be to put it onto the old ride-on and the problem will be solved. Now what to do with all the old mower bits!
I think I'll also persist with the 5HP version.