Many thanks, Martin, and others for suggestions and info.
For reasons that elude me, the 'stuttering' seems to have stopped for the last 24 hours or so, having been an intermittent problem for several weeks. I don't know why, but fingers crossed for no further problems. So, I don't propose to do anything more unless and until - in which case I'll certainly try the manual tuning.
The predictor suggests that I should get everything with a good signal from Tunbridge Wells, and I may in fact be doing so. (If I understand correctly, TW is being or has been upgraded, so maybe this has affected my reception.) But I know that in the past there were some progs that were from Bluebell Hill.