I have the engine completely stripped. The final part bought a bit of bad news. I first found the culprit big end shells. As you can see from the pictures, the fourth big end from the front had excessive damage on the shell and, possibly, the crank journal. The shells were quite swollen on both the rod and end cap side. I have also noticed scoring on the cylinder walls, mostly in the fourth cylinder from the front but also on the third. The others don't look too bad. A re-bore looks like its on the cards.
I have taken some pictures for your opinions/advice but a proper look by the machine shop will be needed for a decision. I think I know what the answer will be though.
There was no ridge on the tops of the cylinders however. Quite a surprise.
The pictures are as follows, from first to last.
Rear two cylinders from right.
Middle two cylinders from right.
Front two cylinders from right.
Rear two cylinders from left.
Middle two cylinders from left.
Front two cylinders from left.
Fourth and third journals on the crankshaft.
Damaged shells.