You need to check which transmitter you are tuned to.
1 Find the transmitter(s) serving your postcode at http://www.digitaluk.co.uk/coveragechecker/
2 Visit https://ukfree.tv/transmitters/tv/Mendip to find the information for your transmitter. The page given is for Mendip Main Transmitter.
3 Check which multiplex and channels broadcast 10 ITV3 and 20 Drama. Mendip transmits PSB1 multiplex with BBC on Channel 49 (C49). Mendip transmits the COM4 multiplex with ITV3 and Drama on C48.
Note that the COM4 multiplex is not transmitted by repeater stations – it is only transmitted by main transmitters.
If your aerial has its bars vertical, you are tuned to a repeater. If it has its bars horizontal, you are tuned to a main transmitter.
… neither are really worth having – you aren’t missing much!