The box might well have been in the same town, and taking its signals from the same transmitter, but it probably wasn't using the same aerial!
Ideally the lengths of the elements of an aerial must be exactly the right length to resonate with the incoming frequency. Digital channels are bundled together into groups (called 'multiplexes') with each multiplex having its own frequency, so you theoretically need a different aerial for each multiplex (in order to have the correct element lengths to match it).
That's clearly impractical so the aerials we use only provide a 'rough match' to the range of frequencies that they're expected to receive.
Your aerial at home is a relatively poor match for the multiplex which includes BBC1 and BBC2, whereas the one at work is a poor match for the one which includes ITV1, Channel 4 or Five.
You might also experience problems, at either location, with signal break-up from other channels in the affected multiplex. The 'pre-switchover' multiplexes are shown here:
http://www.bbc.co.uk/...nfo/multiplexes.shtml
The 'post switchover' multiplexes are here:
http://www.bbc.co.uk/...multiplexes_dso.shtml
The solution is simply to increase the signal strength. Theoretically you should get a better aerial but (since you already receive most stations without problems) a simple signal booster should do the trick. Tesco and Asda sell them for under a tenner but this one, from your nearest Maplin store (or online) offers better 'gain':
http://www.maplin.co....gle&u=223504&t=module
Chris