Thanks for the pointers @andyfrost, @sidevalve5.
I’ve checked spark is OK and had the engine running briefly when dropping some fuel down the carb.
Initially there was a horrific screeching sound but after cleaning the starter clutch and oiling the tiny felt pad on that, it now runs for a few seconds without any awful sounds.
I’ve got a new diaphragm and a couple of other parts that were missing from the carb so hopefully I can get those fitted this week and get it running properly.