Mike - thanks for that. I have two large SEYS Family Trees which I cannot link; I think it should be about 1670/90. The 'Main Line' SEYS trace their family back to ~C11th but my husband's line goes only back to 1720s in Tidenham, Glos. So Yes, I do have info on Gloucestershire SEYS but, alas, not on the right John. I have tried IGI to track him but with no success. I think it more likely that he is linked with the 'Main Line' than with mine. I think also that the connection is earlier than 1790s. SEYS is of Welsh origin meaning 'stranger' or 'Englishman'. It has been used since about 1400 and prior to that as Sais. The family originated in Glamorgan, spread into Monmouthshire and then into Gloucestershire. To date, I have found no link with Belgian or French Seys.
I wish I could be of more help - I'm in the dark too! Judith Seys