Just an update, I finally got round to replacing oil and filter and when I removed the old sump plug this is what I found...bodge job or what.
I'm thinking whoever did this threaded the threads in the sump pan which caused the plug to not screw in tightly. By adding this make-shift washer allowed the plug to screw in by not catching the threaded thread.
Luckily the new plug had threads all the way to the end so I had no issue ensuring it was tightened properly.