Making crumpets in advance?
You are better off making the crumpets in advance and then reheating them. The rising agent in crumpets is usually bicarbonate of soda or baking powder, and so the batter can't be 'held' in the fridge like a yeasted batter - it would be very dense if you left it too long.

So, make the crumpets, then reheat them. I'd suggest using the toaster - this is how crumpets are sold and prepared in the UK. Make sure you have plenty of salted butter on hand too!