Coolant Flange replacement

Guys, I've finally got all the parts I think I need to replace the broken coolant flange on my 1.8t. I've stripped down the engine as I am replacing the thermostat housing as that is also broken, and replacing the hoses as they're brittle, soft or oil contaminated.

I am having trouble getting the the flange however, and the only way I can see to get to it is to remove a part at the back of the head, seen below.

Any ideas how to get this off please?

Thanks.