Hmm.
It could be a bit tricky to get a refund as your right to one would depend upon exactly what you said when you took the car in. A service consists of a certain number of fixed tasks (typically involving oil and filter changes and perhaps some lubrication tasks, etc). Anything which isn't on that list doesn't form part of the service, so the garage is entitled to ignore such things as rusty exhausts, corroded sills, leaking hoses, etc as long as they're not on the list of tasks for which you're paying them.
So if you only asked for a service, and simply mentioned a possible oil leak in passing, the garage wasn't obliged to do anything about it. However if you said "Please service my car AND fix the oil leak as well", then you'd clearly have the right to expect them to do just that. (Equally, the garage could charge extra for sorting out the oil leak, as that task didn't form part of the service).
So your legal rights are, at best, 'a bit vague'. However seeking a refund might still be worth a try!