As promised, the result:
I checked for blockages in the pipes, all clear. I opened the lid and sucked out all the water, absolutely clean as a whistle. Took off pan connector, no debris, really clean. It didn't look like anyone had ever done a no.2 even.
No simple fix so rather than stripping the thing down and still not getting it sorted I passed the customer on to a local Saniflo engineer.
I didn't charge the lady although she argued that she should pay me for my time. I accepted a jar of home made marmalade in the end.
Just arrived home and her sister phoned me to say she wanted me to replace taps in her 2 bathrooms and replace cloakroom suite and added that she doesn't expect it for free and will obviously pay the going rate.
It just shows that there are some people out there who don't expect something for nothing and it's worth doing a little favour now and again.
Thanks again to all who gave advice, even the daft ones gave me a laugh.
