I recently bought a set of "AERO-MATE Brass Rear Deck Screws" but until I knew the thread size I was reluctant to blindly drill into the deck of my Tiger 1 so I E-Mailed "AERO-MATE" and they have been very helpful and the size is-
M8-0.75
If you do not have this in your boys own box of Taps & Dies it is readily available on the www for under £5.
Shaun.