Be careful with interpreting your signal meter Marc. Those things are broadband – it’s not necessarily reading the signal coming from Preseli. It could be picking up anything from DAB radio to 5G and beyond. Thinking of which, have you had a new phone mast put up locally?
It’s far more accurate to swing your aerial 15 degrees either side of where you think the transmitter is and rely on your TVs signal strength readings to steer it into position.