Bathroom project - Drainage

So I put in the new sink, tightened up all the joints on the trap, and eventually got the little leaks stopped.  PTFE tape is a wonderful thing.  Uh-oh, the sink backfills with water when it should be draining.

The old sink worked fine, so logic tells me that the thing that has changed, the new trap, must be causing the problem.  Extensive testing of the new trap - no problems there, despite the evidence.  I tired blowing down the drain pipe - blocked.   Sucking seemed to unblock it (a nausiating experince, to be tried only in dire emergencies).  But still blocked.  There seems to be a trap in the pipework in the wall.  Who ever puts a trap where you can't get at it?

I think that what happened is that the drainage pipe had been unused for several weeks and had dried out.  The crud that normally sticks to the sides dried and flaked off, sending flakes of dried crud to block the trap.  The problem was eventually fixed by sending down some viscious granulated drain cleaner that fizzes violently when wet.   Sudden glooping noise from somewhere in the wall, and the sink starts to work.

We now have a fully functioning bathroom - shower, bog and sink all work.  I have to fit the loft hatch (the tower has a conical roof that houses the TV aerial)  and a little tray for holding the shampoo and other gunk in the shower.  Then it's done.  This is the last post on the bathroom project.  *phew*