Option 1. Take the box upstairs and use the set top aerial to tune it in to get frequency ch 48 (Local TV mux) from Sutton Coldfileld. (Then don't retune again to perform tests on the outside aerial system).
Option 2. Take the set top aerial and try that on the main TV/box ti see if f.ch 48 is found via that (manual tune is better probably?). Although set tops work better upstairs than down and this option may well fail.
Then: back with main roof aerial:
Go over all connections with a fine-toothed comb, re-making any hand made ones. Try alternative aerial fly leads. Is ch48 now OK on the STB and/or TV?
If TV has f.ch48 tuned in via set top aerial but it's not working on the main: Power off the set top box at mains and unplug all hdmi cables from TV and STBs (to be 100% sure no signals can be passed in/out of the hdmi leads.
Connect aerial direct to TV set.
Does ch 48 work now?
hdmi to aerial cross-interfernce can affect one UHF frequency in some instances. Try other hdmi leads with better screening and/or better screened UHF cables and keep the two as far apart as possible.
Use of the signal level and quality metering of the STB and TV can be useful, but needs to be done on every multiplex frequency to give a full picture of things.