The monoblock will pick up two satellites that are approx 6 degrees apart, like Astra(19.2E) and Hotbird(13E) for example.
It will not pick up 28E and 13E, as they are not 6 degrees apart.
It sounds to me that your dish is pointed at Astra(28E),hence getting UK channels using one part of the monoblock, but since the other part isn't pointed at Hotbird, it won't get it.
Is is Astra (19.2E) andHotbird(13E) you wanted?
Or was it Astra (28E) and Hotbird(13E)?
Both are possible, but you can't use a monoblock for the 2nd option, you need TWO separate LNBs.
Robbo