Yep, thanks for the quick reply. Let's just say DOC.
AIP sec 2.18 (aerodrome specific) does give the DOC for ATC units like Cambridge etc, however all of the licenced airfields with A/G radio only don't specify a DOC. Like you I came across this:
"In order to reduce the possibility of interference pilots should be discouraged from calling an aerodrome at range/heights exceeding 10nm/3000 ft."
But "discouraging pilots from calling" is somewhat different to an actual DOC. I belived the DOC for most A/G stations was 25NM. But I cannot source the info to back this belief up!