Thank you both for your replies, I have looked at the IGI and pilot search for Great Barrington, but only births for the period seem to be listed, I cannot find any marriages.
The marriage of William Colliss to Mary Kendall 5th May 1751 Great Rissington appears to be too early, and I had discounted it.
I did find another William Collis Birth which is a close fit, who was born 1751 Minster Lovell Oxford to a Deodatus and Ann Collis, but again cannot find any marriages
I found an interesting document online called "A poll of the election of a knight for the Shire County of Gloucester" 6th May - 17th May 1776 Page 81 under Slaughter Hundreds, Little Barrington where Robert Collis is listed as owning Land.
Dave