Thanks for all the advice and tips, much appreciated.
I'm 100% sure the servo is holding vacuum OK, when I replaced the servo hose I removed the manifold end first then the servo end which was very tight, when it finally came off there was an almighty hiss as air was drawn in, the car hadn't run for over 24 hours so that's pretty good.
Even if the rears are doing nothing I'd expect more performance than this, like you say most of the weight is at the front, the pads and disk's could be incompatible though I've never experienced this before and I felt no improvement changing the pads (motaquip) but it does feel as if all is functional as it should be up until the pads grip the disks and here it feels as though there's not enough friction.
Maybe worth a shot?