Well after trying 3 different sizes, all with different spline counts I had no luck. I visited my local mechanic to see if he could help out.

He used a similar tool to those irwin bolt grip's, but it looked like it was made from proper metal this time
Few serious thrashes with a hammer, breaker bar on, half a turn, and the top of the bolt just fell out! The hardened part with the key pattern in the top is splined into the bolt! So we lined it up with the splines slightly out and smashed it home, one more turn and out it came!

What a saga that turned out to be!