[Ed_Dinning]

Hi Peter, further thoughts on this one to improve performance etc.

There is an excellent book "Alternating Current Bridge Methods" by B Hague, very much the standard work years ago. This gives all the tips and tricks needed to construct a stable and repeatable bridge.
I'm a little surprised at the possibility of stray capacitance problems at the frequencies involved but these can be over come with screening/ neutralisation.

The accuracy can also be improved by using better accuracy resistors, but film types may be inductive due to the trimming spiral used to manufacture them. For the low values required they can be bifilar wound using nichrome or similar wire nad set accurately, or more important, balanced.
A modern electrolytic should have a low ESR for the C4 position, but suitable plastic film caps could also be used in parallel. If mounted close together, and "ends about" it should be possible to cancel out the inductance of the film windings.

Use of a mutli turn pot and /or a small ac voltmeter (as Sussex) for both signal and GB voltage will allow better setting.
C3,5 and 6 should be low inductance types and preferably matched.

There are also techniques to use a coarse/ fine balance control to give a more accurate balance.

Ed