I thought I'd replace the core plugs while I had the head off and I'm beginning to regret that now! I first tried drilling a hole and putting a self tapper in there before using the claw hammer to try and lever it out. All it did was pull out the self tapper and make the hole bigger! So after trying a bit more with larger screws and the same results I realised I was in for a long slog. After copious amounts of levering, grinding, bashing and swearing it came out looking like this and the hole it came out of is completely undamaged which is great because if you damage the seat they never seal up properly again!