Just to update the above. I picked up the HD6670 today and installed this afternoon - a painless install TBH.
What a difference! The picture looks superb and no pixelation at all. I have tried the limited Freeview HD channels and all are performing flawlessly, bar a couple of dropped frames. I have had the athletic championships on for some time - rapid movement and whole screen rapid panning and it played great.
The bonus was the shop I went to was offering a 10% discount for today on the graphic card I was looking at, so enough in the budget to have a couple of pints too