My Perkins 4108 takes a FRAM CH836PL1 oil filter. A good autoparts shop will find the equivalent for you from this number. Don't waste time trying in Halfords though!
I think the perkins part no. is 2654403, but i'm not 100% on that, by the way, the perkins filters are so cheap, it's not worth buying anything else, and i have known 4108's to be killed with a spurious filter (although they were pretty tired anyway).
If you need to know where you nearest dealer is, phone Sabre Engines on 01202 - 893720, or they can ship a part to you.