Thank you JohnS for stating what I have slowly realised is the truth, its a black art form.
So what are we hsm's to do, we do not have the resources to test and must rely on those who have gone before but we bocome stuck if we change to what we assume to be for the better one part of the system only to find it isnt quite right!

Anyone know where I can get some reasonable priced encoders in the UK(only got one arm left and no leg!)

Peter