Hi Richard,
Yes, the jet and needle size for the James are different. The 3/1 on the James uses a No 3 jet and a 3 needle.
Forget the % bit on the needle number. The size numbers on all Villiers needles relates to the % of taper it has and all Villiers needles are very faintly marked with their size but never with the % sign so everyone just talks numbers without the % bit. The jet is clearly marked with its relevant number. No problem there. Enfield adding the % sign was just them being a bit snarky at having to use a competitors carby.
You should have no real problem getting both the right size jet and needle. Villiers made many types of engine all using variations of the same few basic carburettors so the range of needles and jets is vast.
The thing to really take note of when they write of adjusting the mixture in Para 46 is the last sentence where they advise riding at least three to five miles before deciding if the mixture is correct or not. These little two-strokes are very cold blooded and take a lot of warming up to working temperature. Remember it has a cast iron cylinder, not an alloy one like modern bikes and it takes a lot longer to warm up. Conversely it takes a lot longer to cool down once hot so when you fang it round the Great Ocean Road it will take at least two beers to cool down at the Wye River pub.
One final thing. Villiers needles come in different lengths to suit the different size of carby but Villiers in their wisdom made them all the same diameter and numbered them all in an identical fashion. It is VERY easy to install a longer needle from a larger carb in your 3/2 believing you have the right one. This is just another one of those jolly British engineering jokes they did so well.
Cheers,