Hi

I thought the same too, but after rodding (using the small flexible
metal pipes from b&q for unblocking drains), and caustic soda'ing 3
times, and then using one shot drain cleaner, I am pretty sure that
it's not blocked.

Only thing I can think of is that the 40mm pipe could be partially
crushed under the concrete somewhere. The flow coming out of the drain
end, isn't bad at all, and if I flush just water down the sink, it's ok
(doesnt come up the stand pipe). It's only when bubbles are in it - ie
after washing up, that the bubbles rush out of the stand pipe.

I am going to try this:
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=12794

Hopefully should do the trick.
Fingers crossed- thanks for your advice tho.

Charles