Had the installer in today... not sure of the exact cause but it turned
out that the plastic ring that held the (surprisingly loose) rubber
hose onto the condensing trap had come undone - its fixed and works but
the installer suggests I contact Potterton to see if this is a problem
they've come across before... the pipe coming off the trap was clear of
obstructions / water but he couldn't rule out a back pressure problem
from the drains
S.