Squinancywort (asperula Cynanchica)
“Every time a botanist journeyed from London to Bath, he was tempted to get down from his horse and climb Silbury, as Thomas Johnson had done in 1634, for in 1570 the Flemish botanist De l’Obel had written having been up the mound..this ‘acclivem cretaceam et arridam montem arte militari aggestum’(this steep chalky hill dry hill raised by military art) as he called it.... On Silbury he found a plant blossoming in July and August which seems to have been Asperula Cynanchica, which he called Anglica Saxifraga, the first record for Gt.Britain.
Squinancy is the quinsy,sore throat and this waxy--flowered little perennial of the downs made an astringent gargle”
Taken from The Englishman’s Flora by Geoffrey Grigson.
Note; Squinancywort is similer to sweet woodruff which you can find in woods, but I doubt Silbury still has Squinancy on its slope.