Leaking fresh water from Thetford loo

We have a problem with our Thetford Cassette loo leaking water (clean water) into the shower tray.  We didn't notice it when we bought the van - probably because the water drained away down the plughole.  But,  we put the plug in the tray as part of our 'winterisation' of the van and now see that water has collected in the tray.  Has anyone had any similar experience?