My front brake light switch screw is absolutely knackered and all but rounded, checked online to replace by honda part (93893-040-1207 screw-wash 4x12) and it's cheap enough but they all want around £5 delivery which is ridiculous.

Don't really understand screw/bolt measurements but can I grab an M4 x 12mm bolt from local hardwarestore and use that? I'm guessing they're the same measurements... I won't be going and knackering the thread inside the assembly will I?

Thats right mate, i have little stainless allen bolts everywhere on my bike, m4 12mm will do the job.

Ebay will see a set of say 10 stainless inc delivery for a few quid.

Cheers man! Just needed the confirmation that's how screw/bolt sizes worked really, Can't be dealing with these phillips heads, will slowly replace them with allen bolts as you have.

Just for reference; they are not philips head screws. They are JIS (Japanese Industrial Standard) cross head screws. If you try to use a philips screwdriver, it will cam-out and damage the screw. It is well worth getting some JIS screwdrivers, even if it is just to remove the screws that you want to replace. In the UK you should be able to get the Japanese brand Koken.