You might actually be too close to the transmitter. It's possible that the signal is 'swamping' the circuitry in your digital tuners.
If so, you'll need to connect an attenuator between each aerial lead and the TV / digibox:
http://www.maplin.co.uk/module.aspx?moduleno=2 23051
The cause of your recording problems seems to be obvious: The video recorder is trying to record a signal from its own built-in analogue tuner rather than from the output of the digibox. To record that output you need to ensure that the recorder is set to record 'AV' rather than to any numbered channel. (Some manufacturers label 'AV', on the video recorders handset, as 'In', 'Line In', L1', 'Ext', 'Aux' or similar). You then select the channel by using the digibox's handset (and ensure that it's left switched on). Because you're (presumably) using an analogue recorder together with the digibox, you've only got one digital tuner in the system, so you won't be able to watch one digital channel while recording another.
Chris