Confess the word swallet was a new one on me; got me thinking though - which 'swallow' word does the Swallowhead take its name from? Dames assumes it's the verb, and weaves that into the sexual symbolism of the Silbury landscape. But I'm not so sure, 'swallow-tailed' means, '...having a pair of projections suggestive of a swallow's tail, as in swallow-tailed coat...' (Chambers). A glance at a map of the area shows just that configuration, with the Winterbourne as one part of the tail and the Kennet as the other (with the Swallowhead Spring as the head).

Then again it might be a happy combination of both words and both meanings :-)