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Well, after changing the m/c, finding a new reservoir and bleeding it all round at least five times it's now driveable again although I still don't think it's as good as it should be. Sorely tempted to look into the twin master cylinder approach. How difficult was it, David?

Geoff, have you checked the dead stroke between the pedal and the master cylinder? If there is too much dead stroke it can be:
a) a problem with the servo
b) too much play between the outgoing servo pushrod and the master cylinder. If you remove the master cylinder (keep the lines in place, they are flexible enough) and put something like a small washer on the pushrod, so it pushes the primary brake piston a bit upwards, the brakes will engage much better. What happens normally is that the primary brake piston has to move beyond a small venthole before the brake fluid starts to engage the brakes. This venthole vents brake fluid into the reservoir. It must stay open in rest position else the brakes will seize when the fluid gets hot (a common problem, earlier reported). But if it takes too much travel for the primary piston to cover it, precious brake pedal travel and fluid is lost. I removed my brake servo and installed a longer adjustable pushrod. Without the reservoir in place I could adjust the pushrod by checking the position of the primary piston via the ventilation hole (I stuck a needle in it). Now I have sharp reacting brakes.
Ed complained about the brakes of his blue coupe as well, so he did the procedure with the play adjustment and it really helped.
This sounds a bit complicated, but if you look at the pics in the Haynes I hope it makes sense.

Hi Geoff,

Happy to help. Yes the twin balance bar system is really easy to fit. Just like it does on a mini, except for fabricating a plate to cover the hole where the master cylinder fitted. KAD supplied the mini parts which you can see on there website. Like Hans said, the shaft from the pedal to the plunger under the master cylinder is a problem. My system also has a bottle screw type adjustment, so you can take up the slack. I took up that much slack my brakes were binding to begin with! Fixed this by backing off a touch. You will need to convert front calipers to single line using a conversion kit like the minis do, or just buy some KAD 8.4 vented caliper disks, which are already single line. (makes brake bleeding a 5 min job as there is only 2 nipples on the KAD calipers and you are only bleeding either the front or rear circuit at one time, separately).

On the twin system you can buy different size master cylinders to suit the way you want your brakes to pull up front to rear. Have them adjustable via a cable adjustment (for racing only) and have remote reservoirs if you want to.