Freeview reception is not guaranteed for everyone, it was always accepted some properties would have challenging reception conditions. Perhaps you are one of them.Just cannot trust freeview anymore.
To experiment, I recorded a few ITV4 programmes last evening and overnight, no problems at all.
Checked signal strengths at random times proved little or no variation. Quality 100% on all.
Ch 23 73%
Ch25 80%
Ch26 77%
Ch28 75%
Ch 29 71%
Ch30 68%
Not sure if this is any help at all, but as stated above, no problems with freeview from Caldbeck.
Hmm. That is a bit odd. Ch 30 is a 100kW tx so for that to be showing a lot lower than 29 (only a 50kW) seems off. And both are much lower than the rest, except 23 which is another 50kW job. I'd suspect your aerial may be a bit peaky or not optimum for the new frequency spreads in your area post-DSO. It might be worth trying a different aerial -maybe something wideband.