No it isn't the same bird. The bird I saw had a streaked head but the wing colour was the same as the yellow legged gull in the picture. I should have pointed out that it was definitely not a lesser black back. Unfortunately whilst I got a clear enough view of the bird when it flew in it then flew across the sun and I couldn't make out any other features. Unfortunately whilst I had taken my camera I had neglected to take the battery. I have read that herring gulls occasionally have yellow legs, so I cannot rule this out.
HG's have more streaks. With the description you give it will most likely be a YLG as the back colour would be too dark for a HG (I THINK). HG's are much paler in general. Also I think YLG's are bigger than LBBG's and HG's in general.
I just wondered with the description you gave it might be the same that turned up at Pennington Flash.
Talking about Gulls - do any roost at Ashworth Moor res?
Yes there is a gull roost at Ashworth Moor Res. If you check on the Manchester birding forum there was a post about this recently.
With regards to the possibility of my bird being a yellow legged gull, I did think it was at the time, but I have to be honest and say that I didn't have long enough to view the bird in good light to note all the key features and am not expert enough on gull id to be sure of what species the bird was.