Adrian,
if you, like myself, don't want to have Allen screws on your bike, then you will find that normal cheese head screws won't fit into the covers, as the head diameter is too large.
De Groot in the Netherlands has complete screw sets for the timing side and the primary chaincase cover, with slotted heads, correct head diameter, length and zinc plated. I think they also sell Allen screw sets if you prefer these.
Go to
www.degroot-bsa.nlI can't link the sets directly, but you can search for the order numbers:
sch.10screw (timing side set, slotted)
sch.11screw (primary set, s'arm, slotted)
I haven't seen such sets (at least with the slotted screws) elsewhere, and I think the price is okay.
Cheers, Markus