Thanks Chuck, I hadn't thought of the water sitting in the pipe, The ideas you have mentioned are all great and something I can be working on, we could get lucky and the issue could be right near the loo my worry is that is in the wall behind the very expensive kitchen units put in last year, one does wonder whether they have managed to do something to a pipe in the wall during the fitting?? whatever way its going to be costing me a few quid to get sorted which all having only been in just over a year I could do without! Many thanks for your ideas! I will order a borescope, it looks to be a useful bit of kit to have anyway.Another issue, it's bad practice to cap pipes in this manner without also isolating them from the supply at the other end. This is because you create a 'blind' section of stagnant water that can act as a source of infection.
One trick that sometimes helps is to feed an electrician's fish tape into the plastic pipe and then use a pipe finder to trace its path. A borescope can be helpful too as it allows you to look inside cavities through a relatively small holes so that making good is a job for a decorator rather than a builder.