Well.
I'm a lot happier now. Swapped the Talley rings front to back, as per earlier suggestion, and now it is bore sighted at mechanical zero!
Probably should have done that first off and can't figure out how it is so much different.
I had lapped the rings when i first set the scope up before the barrel was changed. They must have been swapped from back to front during the whole work process and i believe lapped rings MUST stay in the same configuration after lapping.
Anyway, I'm not going to argue with the outcome or over think it.
I'm just very happy that it all seems to be rectified.
Thanks for the advice and ideas for trouble shooting. Kept me a little bit sane during my mild panic!
Now i can confidently go about the whole load development process.
Thanks again to all.