Hi @Marc, Is this a recent problem or have you always recieved a mixed bag of signal strengh and reception.
Do all the other caravans on site point to Preseli as I would have thought Moel Y Parc in Flintshire would have been a better option although I don’t know where your caravan is located to give a definitive answer. Where as Moel is a main transmitter Preseli is not extended to Freeview HD on com7 and 8 which explains the reason you are saying SD rather than HD on all channels. Depending where your caravan is you are likely to have poorer signals and/or interference from other trnsmitters or mobile masts, which I expect to be one of your main problems.
The coax coming from your aerial to the booster, is it hardwired directly or connected by UHF plug (aerial plug) If its via a plug there are inexpensive filters you can by, namely 4g/5g filter and a band bypass filter which would remove unwanted signals from other transmitters.
Its unlikely there’s a breakdown of the coax unless the coax itself has suffered impact or has been rubbing against something for a prolonged period. What is however likely is water ingress/damp causing a short in the box where it connects to the aerial. TV brodcasts are transmitted by whay is known as MUX this essentially means more than one channel can be on the same one. Oh its not unheard of that a bit of rain may solve issues on one channel due to how its transmitted.
Any further questions please ask 🙂