Leak from oil filler cap (Diesel)

Noticed its started leaking a small but consistent amount from around the filler cap, any thoughts on what could cause this?

Checked the oil level and that's fine, the crankcase breather is also not blocked as far as I can see (can blow through it!).

Only thought I had was that I'd lost an O-ring off the yellow filler cap, but I don't remember there ever being one!

STC3699 CAP should have a rubber seal inside it. They get hard with age and leak slightly.

Excellent I was hoping that was the case, thanks.