It's a 7/16ths UNF threaded hole and however tall the boot is approximately. Not to worry, my wife makes candles for a hobby so she'll have some spare wax to drop down there, or some of that expanding foam stuff.
I'm also shattered now as the car is ready for painting, well, as far as I can tell with a day to go
I've just uploaded a series of pics
here. including a 44MB movie.
Andi.