How do I kill the main pumpkin tree? I've killed the five heads and have picked up the objects they drop, get to the pumpkin tree thing and I can't interact with it so I end up getting swamped by the hordes of spawns :( I guess I'm missing something simple but can't see what. Help!

You must stick the more fighters to it; yes, next to where its monsters appear from.
Maybe use some obstacle to protect from other monsters, to give time to hit the tree.
I'm okay this scenario is stressing...

Thanks for the tips, I will definitely try that when I can bear to do it again (rage quit so many times now) but my problem is it's not even allowing me to hit the bloody tree! The quest marker tells me I've got 5 out of 5 pumpkins, I've picked up all their drops but when I stand next to the tree nothing happens :( I'm wondering if I have to kill all the spawns first but that seems an impossible task as they just keep spawning.

Are your warriors hitting nothing at all? Or are they hitting the enemies instead of the tree? Because I think they'll always prioritize enemies over objects (like plants, barricades, or the pumpkin tree). You should try and place one of your units adiacent to the tree without any enemy approaching him. Hard, but not impossible.
If that is not the issue, then I don't know what it might be. Maybe try to verify game integrity, in case a file is corrupted.

Maybe that is the problem, I'll try and get one warrior next to the tree and get the others to engage the enemies and see what happens. Thanks for the tip :)

Better stick the more, not only one, to the tree, and use obstacles (such as flag, trap) to give you time.

I must be missing something as I've had all warriors next to the tree but no damage ever gets done. I've rage quit so many times I've now moved on to other levels and have put that bloody level on the back burner for now :D

As soon you killed the last pumkin, you must rush to the tree, don't spoil time to any other things, don't hit back or take things, run.

Thanks, I will try that next time I can face doing the level again :)