Just an update on the solution...
When I took off all the "balancing" pipes (that's what the local Mercury dealer called them, for crankcase pressure balancing he said) I found the bottom one was split too near the crankcase end.
Mercury dealer couldn't help with the pipe but I went to the garden machinery place in town and their very on the ball parts guy soon found exactly the od/id I needed, £1.50 for 50cm. It was fuel pipe for an Oregon chainsaw. Image attached.
Cheap repair for that part of the job then.
He commented they are supplying loads of fuel hose as the modern unleaded affects rubber hose on older machinery.
Probably worth any owner of similar model Mercury/Mariner outboards replacing this hose during a yearly service rather than waiting for it to split.
Decided to do the carb clean and overhaul while it was off.. will do a separate thread for that.