Some ideas:
Which of Joseph and Lydia’s children is your ancestor? What was his occupation?
Do you know where he lived? - apart from ‘Bradley’. anything more specific like the name of a farm, or a street?
Does the will of Joseph Copestake of 1818 give his occuaption and a specific place where he lived? Or perhaps it mentions in the will a house or land that he owns.
So can you tie in the two occupations? or the places? A bit difficult if it just says ‘Bradley’.
In his will, does it give any clues to the children’s ages? Any given as under 18, or 21? Do any of them have grand children? - in which case they will be older.
I see the marriage of the Joseph Copstake in 1760 was by licence. Do you have this? It is on findmypast but I don’t have a subscription at the moment. Does it give any more information about him? Again occupation and where he lived?
I had a look to see if this Sarah Copstake nee Cockayn died, and he remarried Lydia, but couldn’t see a burial on familysearch - not to say she didn’t die of course and it never got entered in a register.
On the 1760 marriage, Joseph signs his name, (Sarah makes her mark). Is this the same signature as on the 1818 will? - albeit some 50 years later.
If he was born in 1733, he would be quite old to be marrying and starting a family in 1775. Not impossible of course, but he may well have been married before.
I see there is a christening of a John Copstake at Bradley 13 April 1763 on familysearch - it just says father Joseph which is a shame.
Perhaps this was the son of Joseph and Sarah. There don’t seem to be any more children then until 1775, so perhaps Sarah died and Joseph remarried Lydia.
Have you looked for any wills for John Copstake, after 1743 at Osmaston, and Joseph after 1733 at Bradley? again I’d be looking for place names and occupations.
Also Lydia Heaton’s father to see if he left a will. Again, these are mostly on findmypast (though I have come across a few that for some reason haven’t made it!) I always check the Staffordshire Names Index at
https://www.staffsnameindexes.org.uk/default.aspx?Index=Cas well, although their ‘soundex’ facility is not as good as findmypast, so you may need to check different spellings. It doesn’t just cover Staffordshire.
Out of interest, how do you know Heaton was her maiden name if you don’t have a marriage for them?