I think it's 3mm David - most non-FAC ports on PCP's are about that size.
I'd give it a good look over while you are inside. Bodged up transfer ports are often the tip of the iceberg <img src="http://jsramsbottom.co.uk/bbs/e/icon_frown.gif" alt="Frown" width="15" height="15"><!--graemlin:-->