After checking, I have some more detailed figures.

Without the booster, I get a signal of between 40 (which completely fails) and 65 (for TV3, which just about works).
With the booster, I get a signal ranging from 80 up to 100 (TV3). These work fine for all of the time, bar the very rare sound-loss problem and very rare CTV8 artifacts. I think the CTV8 issue is more due to their satellite-to-Freeview link than to our freeview reception, though.

The booster is currently connected at the ariel socket, although previously it was connected to a separate socket (the splitter is in the attic) which worked just as well.