Neighbour's identical mower keeps stopping again, so he says I can have it for spares. Ordered a new diaphram/gasket for the carb, see how that goes. If it fixes it, he can have it back, as I would have had my £5.29 of fun pissing around with it .
Hopefully it sorts it but if you want to have more fun tinkering with it you could whip the top cover off. Although the ignition coils on them are very reliable, they can be cleaned up. The magnets are on the flywheel and the contacts are on the ignition coil. If you take the flywheel nut off you will see a key which holds the flywheel in position. The only time I’ve seen a key break was when the blade had hit a lump of concrete, causing an instant stop on the flywheel. To re-time, just position the flywheel magnets in line with the ignition coil arms, place a playing card (feeler gauge) between the flywheel and coil and tighten the bolts.
Generally though, other than kill switch issues, the coils are pretty reliable.