Thanks chempat - no-one at the moment but you never know when someone will come along.
And thanks Trish. Hannah and Thomas only appear to have baptised 3 children - Sarah, Anne and John (or at least those are the only three who have found their way online) and no Mary.
Meanwhile, there were John and Elizabeth Jones baptising a large number of children in Bishops Stortford including a Mary, but no Sarah.
I have also found a reference to Sarah Jones, later Woodham, and Mary Jones witnessing two Flack family marriages in Thorley, which is just outside Bishops Stortford. The first marriage (John Jones to Mary Flack) is in 1763 so the Mary would be of an older generation. The second marriage (Elizabeth Flack to William Hawkes) is in 1783 and by then Sarah Jones was Sarah Woodham.