Interestingly my parents receive a reflection of Emley Moor off the far side of the valley with an aerial pointing away from Emley Moor. The valley is too deep for direct reception. Normally the signal is fantastic, the aerial is on the back of the house below the gutters to screen the signal from impulse interference from the main road at the front (impulse breakup was terrible before I did this).
But in bad weather (mainly heavy rain or high winds) HD reception has bad breakup while SD continues fine unaffected. This is strange because with equal signal strength (which these have), HD is usually a more robust signal.