Just to update you all, I spoke to Mike at Metric via email. Very helpful.
Ordered the part number he suggested and it fits.
One thing interesting to note is the part number he suggested (Elring 046.451) is exactly the same part that I already had.
It was like the whole gasket was cut too far forward as it was definitely overlapping the pistons on the trailing edge.
Anyway, head is on, all good.
Thanks Doug