I have no homing switches on my lathe so I use a slip gauge between the chuck and the platten type toolplate . The problem I think I am having is trying to set the machine coordinates to 50mm . When I get the slip fit , I then set the "Z" home . This puts the DRO to zero . If I try to type in 50 mm directly into the DRO , it goes straight back to zero , but part coordinates are then set to 50 mm . I have tryed setting the Homing/Limits G28 to +50 and then -50 and back to zero again . What I want is the machine coordinates to be set at 50mm when the slip fit is achieved and for the machine coordinates to verify this instead of zero .
Any ideas on my situation would be gratefully received . Thank you .

Have a Home Off value of 50 and it will set the machine coords when you press the Ref Z to 50. This is done from Config menu then Homing and Limits.
Hood

Thank you once again Hood . Just tried that and it works as you said . Spot on .

What should I do with the "X" ? If I use for instance a 100 mm slip to get my top slide to its biggest diameter , do I then put in to the "X"Home off 200 or 100 ? I presume it will be 200 ?

If you are in Dia mode then yes it will be 200.
Hood

30T00,
Here is a link to some info on homing a lathe without switches FWIW:

http://www.machsupport.com/forum/index.php/topic,11502.msg72935.html#msg72935

RICH

Well done you 2  . I have now got it sussed thank you . I love the great support on this forum . I am slowly but surely getting my machine to a happy place for me . I program , set and operate several different machines at work and slowly I am getting closer to the point where I can use "my" little machine as well as I can them , thanks to everyones help . I have got a few more details to sort out , and then I'll be as happy as a pig in *#%t .LOL.

Hood

Happy we could help and thankfully we have have some good folks in here like Hood. 
RICH