I have been offered a brand new Keihin Carb fo my sons 2010 125 , what are the advantages or disadvantages of fitting this part . What problems will there be getting the jetting right ?

Disadvantages are that it does not fit the standard reed block so you need to buy the adapter from a Gas Gas Dealer. You need to set them up as follows

Main 118, pilot 42, needle JJH with clip on middle, 3.5 slide. I think that you can also try the 115 Main but that's a good starting point.

Advantages are that they run sweeter and pick up faster and with more power thru the rev range.