Reason I asked whether you have any other lens was to test out the filter (assuming its the same ring 52mm or 62mm ?) on the other lens.
If it produces more or less same unsharpness result then it confirms the UV filter is 'bad'. An alternative is to go back to the store and test it on some of the lenses there and if the unsharpness is still pronounce, maybe get the store to give you a new one
I should kick myself firstly
(kick - ouch) for forgeting the other post !
Faster lens ? I think everyone would love to have one - but its always the wallet that speaks
and mine is not talking to me at the moment !