I currently have a Shimano XT hub laced to a Mavic 719 rim, however I am shortly getting some Pikes off a mate and will therefore need a 20mm hub Looking at getting a SLX hub, will the existing spokes be the correct length, i cant find this hub on spoke calculators. I have a friend who will build them.

Depends, evey 10mm of flange diameter increase shortens the spoke by 1mm. Probably ok.

The measurements of the SLX hub are 52mm spoke circle, 36.4mm centre to right flange and 25.2 centre to left flange. Stick these into the DT Swiss spoke calculator to find the correct spoke length.

I reckon for a 32 spoke hub you need 257mm left and 258mm right on the SLX where as a HB-M775 centrelock XT hub would use 258mm left and 260mm right.

Thank you both very much for your help! Look like it would be easier to buy a new wheel!