Couldn't find suitable one second hand, so had to bite the bullet and order new one. £325 for the complete cluster, including harness and bulbs. Fortunately a friend does a lot of work for the Nissan dealer in question so got some discount. All back together and working again.
When I took the old cluster off, a fair bit of water came out, no0 idea where it got in as the locks for the bulbs and harnesses were all in place and in lock position. However the fault was pretty clear.