James Salisbury wrote:
Go find a scanner, I seem to remember that the new generation of the CM67
are on a higher frequency band, 868 MHz cmpared to 433 MHz...
http://content.honeywell.com/UK/home.../cmx/HC026.pdf

could this be the problem?

Thanks for all the suggestions.

I got a friend to email the very efficient Honeywell German customer
service and they replied within a few hours. They're a lot more
knowlegable than their British counterparts when it come to CM Zone. It
turns out that my HC60NG/R6660D boiler control has software that is too
old to function correctly with the CM67z and HR80. You can tell which
software you have by looking at the code on the NEC microcontroller on
the circuit board. It needs to be version 1.82 or more recent and not
1.88 (perhaps a typing error as the version numbers are not
increasing?) . As it is impossible to update the software, the only
solution is to buy a new HC60NG. Mind you, nowhere on the Honeywell
site does it hint that not all HC60NG are compatible with the CM Zone.