I have just had a response from Oxfordshire FHS, the gentleman who picked up my email is himself named Simpson and comes from Liverpool where I now live, so he really put himself out for me and searched for all sorts of stuff, more than I asked him for, what a guy!
Anyway, I can tell you that there is no record in Oxfordshire of a marriage between William Simpson and Susannah.
Ann Simpson married Thomas Heritage in Glympton in Oxfordshire in 1817
From Alan
14 Sep 1817
By banns
HERITAGE Thomas(x) o.t.p.
SIMPSON Ann(x) o.t.p.
Witnesses Ann(x) NEIL, Jamed(x) NEIL
(N.B. Although the typescript gived Jamed as the second witness name, I would suspect given the proximity of S and D on the keyboard, that this is probably a typing error for James. As you can see from the (x), neither Thomas nor Ann, nor their witnesses was able to sign their names. In Ann's case, she had the excuse, as indicated by later cesus entries, that she was blind.)
And yes, he had looked up the census and provided me with the entries I found earlier tonight, as well!
Ann and Thomas Heritage also baptised a son, Thomas in 1817, according to Alan, in Glympton. He did not give me a date for that, but with a wedding in September and a baptism before the end of the year, she was obviously pregnant at the wedding, third time lucky, getting the father to marry her! She would have been 44, a hell of an age to be having another child.
Alan reminds me that Ann was blind, but I am inclined to think that may have been late developing, through cataracts for instance, I dont feel confident that a blind woman would have three children to three different men.