Phil, sorry for delay in replying. Here's what I have from OPR Marriages Paisley High Church: John McFadyen in Barony Parish of Glasgow and Jean Renfrew in this Parish (06/09/1806).
Then for 28/05/1782, OPR Kilninver and Kilmelfort, baptisms: Donald MacFadyen Dyer at (I cannot make out the name, starts with a C and ends with aoil) and Janet McLachlan his spouse, their twins was called - his son John, his daughter Elizabeth.
This is the only John McFadyen I could find in a search from 1770-1785.