Steve,
Even new plugs fail, quite often at that. When I was an agricultural engineer in the Sixties, we still did quite a few petrol paraffin tractors and combine harvesters. When drawing new plugs from the stores, before travelling out to the farms, we would test them on the test rig ( either Champion or AC tester), it allowed the plug to spark and as the pressure was raised to simulate the cylinder pressure, you checked to see if the plug was breaking down. It was not at all unusual to have (Champion!) plugs out of the packet, failing. So it is worth swapping plugs if you have any spares.
Also, just had a thought, perhaps the plug grade is not quite right. A problem with 18mm, as there is not a large range available in this size anymore. Did you select C5 as being nearest to correct, or is it the only one in that reach that was available? The grades run from hot to cold and have a huge influence on running.