I was about to post the question below. I've solved it now but I thought it might be helpful to others with the same problem if I posted it anyway.
''My Oneplus 5 stops charging after a few minutes. I get a little buzz as usual when I plug the cable in, the LED shows green & the battery shows a lightning symbol but mostly after a few minutes the charging stops. Sometimes it does charge fully but often not. The problem persists with different cables and chargers.''
I used a torch and magnifying glass to look closely at the port and it was quite dusty. I gave it a blast with a canned air blower & then used a carefully shaved down toothpick to give it a gentle scrape and it's now charging normally again.
Cheap fix!
David