Oxfordshire wills proved in local courts are held by the Oxfordshire History Centre, and images are available on Find My Past: https://search.findmypast.co.uk/search-world-Records/oxfordshire-wills-index-1516-1857 (but obviously you need a subscription.)
For the will of Robert Clack, (and many other people,) you can find a reliable transcript on the website of Oxfordshire Family History Society here: http://wills.oxfordshirefhs.org.uk/az/wtext/clack_004.html
The main wills site is here: http://wills.oxfordshirefhs.org.uk/index.html This site doesn't contain all the Oxfordshire wills of course. But for those that are there, submitted by volunteers and curated with some care, it is very useful.
As well as local courts in Gloucestershire and Oxfordshire, you would need to check the Prerogative Court of Canterbury. These are held by the National Archives of the UK (who I think are still providing free digital downloads) and through a number of genealogy subscription sites, including Ancestry and The Genealogist.
When it comes to actually using wills and other probate records as a source, here are some hints:
- It's always worth trying as a source - people who one would not expect to find may have left a will - but don't be disappointed if you don't find anything. (It was common to leave making a will until the last possible moment, and many died without doing so.)
- Many of the most interesting connections come from wills of people who are not direct ancestors; this includes people with different surnames, of course. So starting with Thomas Cambray's will, it would be worth checking all the people he named in his will to see if they also mentioned Margaret. (This is another reason the Oxfordshire FHS site is so valuable, because it indexes everyone mentioned.) And some wills are really uninformative; disappointing but true.
- Because a will can be a long document in unfamiliar handwriting I find it is all too easy to miss things when reading on screen, so making a transcript or at least full notes can help to ensure that you miss nothing important. (You will almost certainly come across words that you cannot read on the first (or second) go too - at least I generally do. Move on and come back later.)
- While wills for married women in 18th and early 19th century England are really uncommon, unmarried women and widows often left wills, and because their family relationships are different to their husbands, they can be illuminating in themselves.
- Not everyone mentioned in a will is a relative of the testator, although they are probably all part of the history of the testator. And (particularly in older 17th and early 18th century wills) it is worth noting bequests to the poor of such-and-such a place; they often indicate places of birth or residence. Not solid proof, but useful clues. In the 16th and 17th centuries, bequests to a parish church and diocesan cathedral may have similar interpretations.
Best of luck - you will need some - but it can provide insights available in no other way.