If you mosey gently out of the marina, turn right (as if you are heading for the lock) and just beyond the 5 or 6, 38 footers moored on the hard, there are two floating pontoons. As the river flow is so gentle at the moment, why don't you practice approaching those? See that your fenders are adjusted to be 2" above the water level, or level with the pontoons.

As others have said earlier, I bet the marina staff would be more than happy to spend half an hour with you.