Overheating and need some help...

"cracked the filler tank lid but there seems to be a lot of back pressure there and the water seems to 'chug' out of there" - lots of pressure in the expansion tank and the fact it happened at high revs (motorway) makes me think head gasket. Run it from cold with the expansion tank lid off, have a look in the tank when someone revs the engine - look for bubbles. Bubbles = head gasket.

sounds like a head gasket or a cracked head\cylinder to me.

Combustion pressure is causing the system to leak like the rad and its the blowing and gassing your getting out of the expansion tank

Yep........definitely will be the head gasket, unfortunately very common with 300Tdi. Depending on how much overheating took place you may need to replace the head.

Is the water pump circulating the fluid?
It has been known for the vanes to rot away, which gives effects similar to those posted by the OP.
It it ain't circulating, it's boiling away!
BTW is the gauge reading high?