There have been 7 series. The original few also starred Miranda Hart and Tim Vine and were indeed very funny. Gone off a bit since Tim Vine left I think but still quite funny.
I enjoyed seasons 1 - 4 immensely then it gradually got a bit tedious.
The Tim Vine seasons were the best and I enjoyed the cameo by Bobby Ball as Lee's Dad is some later episodes.
This is exactly the reason I buy a decent listings magazine (Radio Times, arguably the best)...without one, you can end up "flicking through the channels" and missing some worthwhile TV.
It was only by seeing it in Radio Times, that I discovered that one of my "must see" American shows had started a new series on More4...with no prior warning on-screen.
I like Lee Mack, I think he's very amusing and have seen all of the episodes of Not Going Out, but I did prefer the earlier ones when Miranda Hart and Tim Vine were still in the series, and I think that Series 1 started with an american girl living in the flat, rather than Lucy.