Done a bit more troubleshooting moved the plug from No 1 cylinder lower to no.3 upper and the problem stayed with the cylinder (no1) . The plug that came out of the problem cylinder was black and appears oily, is it possible as the engine has just had a tear down inspection and rebuild that the rings for cylinder number 1 are not seating correctly and are letting the combustion past at low rpm. The plug for cylinder 1 was cleaned and now it's fouled up again within one hour of flight.