I don't have channel five on Freeview anymore. It's listed, but just a black screen and no sound. I've tried Installation -> Add new channels -> automatic search, but no result.
Using the 'Add new channels' option tells the box that there's no need to update any stored information about existing channels. If you use the 'Replace channels' option, the box will will reset all of the data for all channels and C5 should reappear.
I used the 'first time installation' option which replaces the channels as you suggest.
A strange situation now. For channel five I get sound but a blank black screen. However, if I press 'i' or 'menu' then the pictures appears. If I press 'i' or 'menu' again, then the picture disappears. So I can watch Five with the programme information on the screen, but not without.