Q: I’ve ordered a jib/genoa halyard replacement for a GP14 this morning

The lengths required are:
A = 5240 mm  Taken from the metal eye, not the shackle.
B = 80 mm

The existing wire has snapped.  I may be needing a helpful hint for how best to thread the halyard back through the mast...

Thank you.

A: As far as re threading the halyard try

1) Remove the Jib Sheave Box on the mast underneath the forestay.

2) Pull the Main Halyard, sail end, fully down and attach the rope on the end of the Jib Halyard to the Main Halyard inside the mast.

3) Then lower the main halyard until you can fish out the Jib Halyard at the bottom, hoping the halyards don't get too twisted!

The only other way is to buy some 3mm 1 x19 SS Wire about 6m to thread through the mast. It is very rigid so is easy to force down the mast http://www.marinesuppliesdirect.co.uk/stainless-steel-wire-1x19-wire-rope-176-p.asp
I hope the above helps.