405 petrol smell update - it's the charcoal cannister, but what to do next?

Hi all,

Quite a while ago I posted a message about my 405 estate (XU7 petrol)
smelling of petrol under certain circumstances. Have now narrowed the
problem down to the system that sucks the petrol fumes from the tank. The
smell only is apparent when a: the engine is cold, b: the engine is idling,
and c: it doesn't happen when the tank is filled up completely. A and b
correspond with the situation where the solenoid valve that takes the fumes
into the inlet is closed and the fumes are collected in the charcoal filter
cannister. C means there aren't many fumes in the tank yet (no smell), so I
guess this might be the culprit. Cannister might be leaky or maybe the
charcoal filter is saturated in some way or other? Could a leaky cannister
also cause an influx of false air into the inlet when the solenoid is open?
Has anybody done any work on this system? Can anyone tell me what to look
for or how to get at the cannister? It's supposed to be somewhere under the
non-driver's side front wing (LHD car, but I can't see anything easily. Can
the charcoal filter be replaced, or does the whole unit have to go? Any
idea of the cost?

Many TIA