Hi Sue
Well after an extensive index search I'm sorry to report that I cannot see the first marriage for William Gillard.
As regards Harriet Gillard, I searched through all the marriage records and could not find anything. She is there in 1851 but gone by 1861. I thought I would look through the death records from 1851 to 1861 and low and behold she died 9th Mar 1857 at 04:15 am at Dorset Street, St. Helier (Ref: St. Helier deaths, Dec 1855 to Jun 1857, volume 11, page 94). She was aged 31 years and died of "phthisis". Ann Haley was present at her death. However, it gives her name as Harriet Gillard, widow of Samuel Down. (The 1851 census shows it could be Samuel Downs in St. Clement. Unfortunately it only gives his place of birth as England) So I searched again for a marriage and then for a death of Samuel Down, but failed to find either of them. A quick search of UK marriages also failed to find anything. Could they have married overseas or in Guernsey??
In searching records I have also looked at a Funeral Director's details. They show a funeral on 26th Apr 1892 for Marie Mill Gillard, aged 72 years, widow of George Toms and wife of William Gillard. She died 23rd Apr 1892 at 1, Poonah Lane, St. Helier.
I'm not sure whether I've completed all your enquiries. Is there anything else you need?
Stewart