I now have two replies from Siemens. The first suggested a full base station reset (and yes, they suggest this for everything they don't know the answer to). The second, after I had queried the necessity for a reset of the base, said "...Gigaset IP phones are not fully compatible with the repeaters and that is why you are having this deregistering issue."
Words fail me. Sounds like a case for Trading Standards. Any lawyers here?