Cheers Mark,
I've been doing some research and splicing spectra isn't really that hard, so I'll get some and do some practice. http://www.thecoastalpassage.com/eye_splice.html . I can test the strenght of the job with a winch or pulley set up in the garage.
Also running the new halyard will mean the splice will need to be done in situ, although you said you had yours spliced off the boat? How did you run the new halyard with the splice in place?
Michael