S3 Master brake cylinder ??

Evening Folks. My master brake cylinder started leaking today (runied my paintwork...), the number stamped on it is: 64677249 which translates to part code 90577520 (I hope). I've found one with Mcdonald Landrover for £28
http://www.mcdonaldlandrover.co.uk/p...-64677249.html

Now the question is...... My 71 LWT is a hybrid, built on an '86 (ish) Range rover chassis, so has all the RR running gear including all round disc brakes...

the master cylinder thats on it is for S3 (according to the description). Before I order it, am I best replacing the servo and the master cylinder for a 86 (ish) Range rover one to fit with the everything else.....

Or doesnt it matter about the master cylinder as long as it pumps fluid

Just had a thought, if I changed the master cylinder, I'd have to change the servo and the pedal box....
Oh sod it, will just replace like for like.... It stopped before