Ah, seems I should have read the User Guide, you have to scroll all the way down to the bottom. Not sure why you can't just Force Touch as that doesn't seem to do anything.
"Delete email. If youre reading the message in Mail Mail icon on Apple Watch, firmly press the display, then tap Trash. If youre looking at the message list, swipe left on the message, then tap Trash. You can also delete a message from its notificationscroll to the bottom of the message, then tap Trash."