Engine leaks are so frustrating...
Q Having discovered that my BMC 1.8 engine has lost water and located the area of a leak as shown in the photo, I can’t actually work out where it is coming from. Is there is a gasket or something similar in this area? RICK ANDREWS, via email A TONY REPLIES: I can see a slightly glossy area (indicated by the arrow). This may indicate a pinhole in the rubber sealing cap directly above it. Some of these rubber bellows are leak-prone.
A drawing for the (similar) 1.5 shows a core plug or Welch plug at the back of the engine block. Such plugs perforate with corrosion from the inside. Feel around the area with a bent wire: if there is a disk shaped recess at the back of the engine block, there is a core or Welch plug. Blowing up the photo I can see what looks like jointing compound oozing from where I expect the plug to be, which suggests an earlier repair.
It looks like the engine back plate will have to come off for access, which involves removing the gearbox and jacking up the engine so you can take the flywheel housing off, then the flywheel. Calcutt Boats at Stockton (01926 813757) should be able to confirm the existence of a plug and supply the parts you need.