I have a pumpkin growing tryphid-like in my greenhouse. It is going to
take the whole place over. What can I do?? Can I chop the end off to
stop it growing any further or will this kill the whole beast??

I hope your greenhouse is BIG. My pumpkins have been known to put on 30
feet of stem growth in a season and that's outside! And if you get any
decent sized fruit they could fill the rest of teh greenhouse. Is it too
late to transplant (not that I have any idea whether such an exercise is
feasible)?