Hi barryt.
I have no problem with ITV 1 HD (10832,H,22000,5/6) but cannot get BBC HD (10847,V,23000,8/9). To be honest, HD reception is not an issue with me at the moment but may be when I get a second Humax for my cinema room. I have also heard that the BBC are to axe BBC HD and replace it with BBC2 HD (a simulcast of BBC2) as part of its cost cutting measures.
Reception of BBC1, BBC2, ITV, CH4, BBC3, BBC4, FIVE, etc, etc, is fine 24/7 all year round.
As an aside, I used to be able to get BBC HD during 2010 by pressing the red button when watching BBC1 but that option does not seem to be available anymore. Not sure why.
One thing that I would promote is an Invacom (or similar quality) LNB and a Humax receiver in order to maximise signal strength/quality (that's 2 things
).
Cant help on the Gran Canaria question but like you I cant see why dish size would be an issue between our 2 islands given a clear line of sight.