Hi Victor,
I was aware that the William Wallis that i am mentioning married Sarah Winston and Marry Berryman but on this web site
http://www.glosgen.co.uk/records/minchmarr7.htm you have a William Wallis married to Mary Evans the date is William Wallis & Mary Evans 1 Mar. 1791.
This transcription is taken from Volume 11 of Gloucestershire Parish Registers - Marriages, edited by W.P.W. Phillimore, M.A., B.C.L., and published in 1905 by Phillimore & Co.
From this the children i mention are likely to have happened, the William i am looking for is probably between 1762-1769 and Bisley is likely as so is Winchcombe.
As William married in 1787 and 1799 is it possible he was also married in 1791 or do you think it is just another William Wallis i should be looking at now?
The Mary i have is born about 1766 Avening glos, so the marriage date would tie in well in 1791 she would be about 25 years old.
I think i shall check for death records of Sarah and Mary Wallis about these 3 event dates of about 1787-1790, 1791-1798 for further answers.
Thanks Victor it would be very interesting if you find anything more.