Thanks all for help so far,
just to update......
I have spoken to the shop. Now lifted barrel, but on prior inspection there seemed to be a gap on the front right hand side. The bolts were not overly tight. There was a small piece of sealant (?) protruding from the engine casing (not sure if this would be enough to cause the problem?) and there is no visible damage.
Before going any further, having removed the sealant, I have replaced the barrel and tightened the nuts starting with the one the gap was on previously. This looked a lot better with no visible gaps or day light.
I've once again filled it with water which it held this time, so I guess problem solved? although I think I now need to loosen the barrel once more, remove it from the piston and check the crank casing for water from the first mishap, clean, and rebuild once more with a new gasket?
Would that be the suggested way forward?
Thanks again, fingers crossed.....................