Chris

If you didn't have to change your aerial when you started getting Freeview, it's probably not a wideband aerial. Terrestrial transmitters in the UK are organised into UHF channel groups so that they don't interfere with each other. But when Freeview and analogue were running side by side, sometimes it was necessary to put the Freeview muxes on a channel that was outside of the transmitter's normal channel group. Therefore, in order to receive Freeview from that transmitter, it was necessary to switch your aerial for one that was capable of collecting signals from any of the UHF channels, and not just the ones from within the group allocated to your transmitter.

I believe the intention was that, once digital switchover was complete, all transmitters would revert to making all their broadcasts from within group, but under this new HD rollout plan it looks like that's no longer going to be possible.