Strange, just realised half of my comment is missing of my last post?
Marc a few easy things to try if your at the caravan, before you start shelling out any money.
Manually tune in to channel 27 522mhz see if you can pick BBC up from Blaenplwyf and its quality.
Maybe wort trying…
Remove the aerial lead from one TV and do an auto retune. This will clear the the memory of any stored channels. Put the lead back in the TV and do a manual retune.
Bypass the booster for one TV – at the same time a photo of the cross sectional of the coax would be useful to the type you are using..
What DVB-T signal strengh meter do you have. You could do a quick double check of the aerial alignment.