"Both the BTO and the RSPB say that sparrows are in decline according to their annual surveys. Friends in Manchester and Wendover tell me they have not seen a sparrow for over 12 years. I live on the Nevill estate and daily see flocks of up to 15 individual sparrows; my nest box was occupied for the 1st time since I put it up 8 years ago - by a very diligent pair of sparrows who successfully reared two clutches. I suppose all you can say is that they are "locally common".
The decline in our national sparrow populations, both rural and urban has been a cause of great concern for some time. So it's good to see and hear populations as you walk around Lewes. The decline hasn't been totally explained and there may be a few factors at play.
Your comment that they are locally common is certainly true. I can hear house sparrows in my back garden - they must nest a few doors down from me - but I have only seen about 5 house sparrows in my garden in the past 2 years. They're a real garden rarity for me despite being my neighbours!