Hi Mike,
sorry for the late reply, I am going through a pretty busy period recently.

> 'python-sip-tqt-dbg ' is installed, as part of the build sequence,
> i.e. after each suite/prog is built (tqt3, tqtinterface etc) it is
> automatically installed and the next build sequence starts. Perhaps
> I'll try not installing and let the build sequence find and install
> the dependencies.

If I understand correctly from the above, you build a module/prog, then install it in your system, then move to the next one and so on. Correct?
If so, here comes the next question: which command/tool do you use for building a module? debuild / pbuilder? dpkg-buildpackage? manual invocation of fakeroot debian/rules binary? Others?
Perhaps python-sip-tqt-dbg is not installed in the right environment. For example you could have python-sip-tqt-dbg installed in your system, but debuild may not be able to find it.... or there may be other combinations.

Based on "how you build", let's see what the next step is, even though it seems strange that the problem came up all of a sudden in wheezy (jessie is a different beast, and there is no guarantee that what works now will still work the next hour.... been there many times :) )

Can you also upload a build log? It may help as well.

Cheers
  Michele