It became a case of get their bank to call us and we can authorise the payment. Not as easy as it sounds with the bank being in Germany!
In the end the booking was made through DB UK, whom I found on the DB website. An email and a fax of the train details got it sorted. A cheque for £380.16 went off today.
Yes it is a lot but a price to pay for getting 650 miles each way whilst mostly asleep.
Paying now means there is plenty of time for putting money aside for when we get there.