This might be down to the version of firmware that you're using, the last string of numbers (2471201) denotes the modem code which the router is running.

I think this modem code is optimised for Annex M lines. If youre using Annex A, try 232201 or 211801 instead. I think you can get these versions of firmware from ftp.draytek.com

As a previous poster said, the version I am running is meant to be the best for an ADSL2+ connection.

Irrespective of this I have fixed the problem by swapping out the router. That is what has finally restored a stable connection. I have not previously had a problem with a Draytek router becoming unreliable ( plenty with other makes!) but there has to be a first time I suppose. I will be sending it back shortly for a replacement.

Obviously the error message was rather misleading in that it implied that the connection was being dropped by the other end rather than the router losing the connection, but I suppose that when a product becomes faulty you can't depend on the error message being correct.