Ty, the 25:1 and 20:1 are given as alternatives on the cap (presumably depending on severity of service). Here is a picture of the cap:
[Linked Image]

I've also shown the air vent that is part of the fuel filler cap. It is a tiny bloop tube type, and is a bit prone to getting stuck shut if the tank is left with stale fuel in it for a few years. I think the petrol dries up then the oil oxidises and forms a kind of glue. If it happens, it has to be fixed (wash it in petrol and squeeze it a few times).