After experiencing the problem on BBC3, I contacted the BBC who replied that weren't aware of any issues but felt that manual tuning might solve the problem.
Presumably, they feel that the relatively close proximity of Wenvoe and Mendip may be the source of the issue. Or rather the way the box handles this. (Interestingly, a similar situation exists with Winter Hill and M-Y-P)
I'm not entirely convinced by that, but at least they responded and logged the issue. Which is more than Vestel have done, who seem unable to even acknowledge an email.
I'm not prepared to put up with the problem, so if there is no update on the download schedule I shall reluctantly return the box for exchange.