They're just standard M5, often button head.
You really want stainless steel, and the choice of stainless fasteners in many UK hardware shops is poor. Unless you've got a local independent shop you may struggle to get the right length* despite having a few mm to play with.
I buy mine on ebay, always getting a few more than I need. Buying less than 10 at a time is expensive because you're paying postage (usually included in the price).
* Yes, you can cut them, but if they're going into aluminium you need to do a clean job to avoid damaging the mating thread. If you're buying, buy the right length to start with.