This copy will
https://www.familysearch.org/ark:/61903/3:1:S3HY-67K9-PVD?i=526&cc=2365059Will of Robert Donkin Gent of Belsay, written in 1771 and proved 1781.
Mentions a son, Robert Donkin of Byker
May not have anything to do with 'your' Donkins, but worth adding to to the possibles collection in case anything else turns up to confirm/deny it.
Sometimes, especially this far back, we need to just keep on poking round for bits and pieces that are 'possibles' until we can, hopefully, get to the stage where we have enough to to make an 'on balance' decision as to whether or not its the correct scenario. That can take a lot of searching and a lot of years, but if you keep good notes of what you find and where you still have a bit of hope that you may crack it.
If it wasn't for that little bit of hope, I'd had given up years ago :-)
Boo