Accelerator pump screw

There is a rod that controls my accelerator pump and a little brass nut. I have now lost 2 of these. The original that was on the bike and a second that I bought off eBay. I’d like to buy a locking nut to replace this so I don’t lose another one. Any idea what thread these are? 

M3 x .5

I am in the UK and run pumpers, I use a thread locking agent on mine, I also made a special screw for mine much longer so when the threaded rod is flush with the bottom of the screw it is at the factory measurement, much easier than measuring exposed rod.