Took it up to a mate today who has a decent quality 6 sided plug spanner and couldn't shift it.
Have bought penetrating oil and put some down it today
Went to motor factors who showed me a standard plug in a standard plug spanner and it is not a good fit ! Loads of play no wonder they slip.
Plus because of the slightly recessed design of the sockets they don't actually grip that much of the plug before the socket touches the head it doesn't actually grip all of the plug just the top bit.
To get round this I may grind the top couple of mm off a 6 sided plug socket flat so it grips all the way down
Also ordered a deep 20mm socket as a last resort to try and tap on and grip it
