My Foxsat-hdr, from new, has always showed both signal indicators fully in the green on both satellites so I'm thinking the dish and LNBs can't be too badly aligned despite me changing over both LNBs from single to quads, without access to a signal strenth meter. However, watching the footie on ITV HD for the first time last Wednesday night, I noticed that the lower indicator went into the orange and later on, down to the red. The picture then started to drop out. All the SD channels were still full green at this time.

Does HD reception need more signal? Can someone explain what might be going on?

It was the broadcasters fault, the problem was widely reported. There is no difference in the reception requirements for HD it merely takes more bandwidth.