While I had the bonnet off so that I had good access to do the carbs I also installed a set of kevlar Greenstuff front brake pads.

Being a cynic I didn't really expect much improvement, but to my surprise the sponginess in the brake pedal has gone completely.

To be honest I'm not sure how changing the pads can account for this, and I wonder if in replacing the inlet manifold I have inadvertantly fixed a previously undetected vacuum leak, which has in turn allowed the servo to work properly.  I'm happy enough with the brakes that I'm going to hold off fitting the front Wilwood calipers.