Blimey, I didn't think my post would provoke such a response and reaction... Thank you all very much for your valuable, varied and interesting information. Although, ironically, I am possibly more confused than I was before! Only kidding.... The main thing I have found out, best of all, is that it can my job can be done for less than what Scottish/British Gas are charging.
I'm still not sure whether to go ahead with the Magnaclean and if I do, GPlumb, I would possibly take you up on your offer of £160, as I can do most stuff around the house, but when it comes to gas pipes, I always tend to err on the side of caution. If that is this case, I will get in touch. Going to mull it over tonight and tomorrow.
Thank you all very much indeed, it is very helpful and really is appreciated, all this input, hats off to you.
K