new water pump and cambelt kit - but leaking coolant?

2000 renault scenic rt 1.6 16v automatic
new water pump and cambelt kit - but leaking coolant
the car runs very well no over heating of any kind
temp' is steady and on the button

but now and again
this only happens when the engine is left over night to cool right down
i find antifreeze/coolant leak under the drivers side

i took the car to (by 2 garages)
they both ran the engine for a good while - brought up to high temp'
then left to cool down no leaks were found anywhere
from hoses or around the head gasket or water pump nothing leaking
then

i took it to a renault dealer
to have both a diagnostic and pressaure test no leaks found from the head gasket
no leaks of any kind were found
all i was told after paying the diagnostic fee ££££
it looks to be
either the head gasket
or a
cylinder head problem
yet they agree that the car is running very well

please can you help and advise