I know right! I have 2 guinea pigs! The female has had 2 litters in her life! :)
I'm going to keep my betta in clean water for several days (I'll do more frequent water changes than normal).
If it gets worse I'll do a salt treatment in my spare 14 litre tank for a few days using Saxa Rock salt (when I can absoulutly confirm it's safe).
If it gets worse still (I hope it doesn't) I'll treat him for 5 days with Myxazin Anti-bacterial Medicine.
Thanks for everyones' help! :)