Leak after fitting unvented cylinder

[paulusgg]

Recently I installed two 250ltr unvented cylinders manifolded together as the customer was converting the loft and the water cisterns needed removing. It's a large house with four bathrooms which were all served by three 3bar pumps drawing water from the loft tanks and the old hot water cylinder. As part of the unvented install all the pumps were removed and everything went to a mains pressure and balanced supply. So far so good
However, on filling the system (quite late on Friday afternoon) we noticed water dripping through the ceiling below one of the bathrooms so we decided to shut the water off and investigate further on Monday, the assumption being that an old gravity fed joint couldn't cope with the additional pressure.
When we returned on the Monday and exposed some pipe work in the area of the leak all we found was Hep with no joints. But to confuse things further when the water was turned back on........NO LEAKS

Any ideas? Has anyone come across self sealing pipework?

 

[SJB]

Only ever known leaks to stop themselves on heating pump valves and gate valves leaking from where the headgear meets the body due to the scale forming and plugging the hole. Doubt that would be the case on plastic water pipes.