Do you have any repeats of BBC1 etc in high numbered channels above 800? You may be receiving from two transmitters, one strong (Crystal Palace) and one weak (somewhere else).
To check the signal/quality percentages if CP has not tuned at all, you will need to partially run manual tuning again and select UHF channel 23 with DVB-T and then read the values without actually trying to retune.
By the way, which Humax box are you using, it might matter and there are quite a few?