OK, I've called and told them I'm keeping it.
Yes, I acted too soon in buying it, like an over excited child. Yes, I think my old TV handled SD better.
However, it isn't that much worse considering it is bigger and Full HD. Indeed as I've said, some SD is better than other SD. The same thing was true on a CRT it was jsut less noticeable. The HD and BluRay performance are both very good. Also, the adaptations in thinking I'm having to amke are the same as when I got my last TV, just a little more so, and I spent ages considering the last one, and it was so highly reccommended by people here.
The guy I spoke to gave me a quick explanation of the 50Hz thing, saying it scans x times and you end up with 600Hz or summat...safe to say there is a lot of room between my head and his explanation...
And I can always flog it without guilt now, should I choose to.
I guess the only way to be entirely happy is for everything to be in HD, or for TVs to get much better with SD, neither of which seems likely to happen any time soon.