You say you can see the crud and the lock mechanism is soaking which suggests you already have it off?
If it is off just dismantle clean and grease it (being careful with the catch spring,) it is not particularly complex.
Skippii's OP post is very good
On reassembly the critical thing is to ensure the little rubber seal on the key flap is in good shape and seats cleanly. That little sod is what lets the rain in, not usually the big chunky rubber the bolts go through.
Once again unless fuel theft is a major risk omit the one allen bolt under the flap then you can always unbolt the lock in an emergency. Don't drill unless there is no choice because a thief can easily see the drilling but
not an omitted bolt under an undrilled cap.