Mr P.J.Ball: Yes, you will have to do a full retune on those dates.
I say this because in general it is the case and will definately be the case on 21st March. I have a Toshiba TV which, at first switchover, I didn't have to do a full retune because simply manually tuning in the BBC multiplex (which had moved and switched to full power) did the trick. This often does not work, it depends on the device. However, there is nothing to be lost in trying. If your equipment allows, you could try tuning to channel 24 on 7th March. If that doesn't get the BBC channels back, then you will have to do a full retune.
Rowridge is expected to be back on again by 6am, but don't be surprised if it is back before then.
Many thanks for your update. Looks like an early start on those dates (Three tvs and 2 PVRs to retune)
Will certainly try the chan 24 manual job first and do the full wipe/retune on the 21st to get HD.
Looks like another to do on the 18th April!