Further to my earlier post, and after a number of e-mails and telephone calls, Sky finally agreed that there was probably a line fault and they would get an engineer out to check the line.
He arrived the following morning and after a bit of too-ing and fro-ing around the house said there appeared to be a fault about 500m away, ie in the cabinet. It turns out the cabling from the house to the cabinet is copper, and from the cabinet to the exchange is aluminium - a hairline crack in the aluminium wire. I now have a silent - non-crackling - telephone and my download speed is just over the top end of the range i was quoted as likely when I took up their service.
Happy with what I have now, annoyed that I had to bang on the drum for two years to get a fault fixed.