I am glad the new online account viewing has proved useful. I am confused why RMG has told you that you cannot adjust the monthly payment. Its probably more to do that they don't want to do the maths to estimate what the payment should be.
I too thought the admin charges were a bit much, but then to play devils advocate you can understand RMG wanting to try to generate some revenue from this new service, as I am sure the capital outlay was quite a bit.
Anyway, my solution would be to just make payments via internet banking with the details they provide. This is the approach I have applied for paying my water.
All the best