OK, OK. Roughly 42 hours of that silly darned message and it has worked of a sudden. Thanks for the two numbers Sheila, they are (of course) only ever engaged. I had emailed and have an answer of the nature of 'there have been some difficulties owing to the number of payments being made, this has now be dealt with and normal service is now available'. I got outta there post haste and paid toute suite, to my relief.
However, to all newcomers. Do not forget to print your receipt. To PDF if you can, to file anyway and a paper copy to have handy. Just in case.
N.B. I have never had a single problem before. So this is perhaps a glitch. I actually come out €1 better off using their online calculation as well.