Some v helpful info, much appreciated.
Fretting hand position has been picked up from lessons tbh Mog, but I always stand when playing at home & at lessons, so would have guitar at same high level pretty much all of the time. Have tried a few different strap lengths - too low & I felt that I was overstretching, and too high & felt that I was cramping my entire upper body up, so have settled for a happy medium. As you said, teachers don't know everything & my teacher is not a specific bass teacher, so I think he sees more guitar players than bass players - he's still sooooo much better than me though
From reading some of the other forum queries, I have purposely tilted the neck higher than before as I did notice that I had my fretting wrist at a more acute angle than was comfy, although from doing this, have noticed a bit more neckdive on the Jazz - more experimentation required I reckon! I have also tried to not grip the neck quite as much with my thumb, which has also helped to some extent.
Also tried Essexbasscat's theory using my mobile & glasses case (which is about twice the width of the phone) & did notice that my thumb was almost level with my index finger with the glasses case, but with the mobile was almost level with my middle finger & had a more pronounced angle on the joint where the thumb meets the hand - might be why my thumb joint is more painful?
Might try your suggestion Charic and pop into the Andertons near me to try a number of different neck sizes against the Jazz. As I said to you on Saturday, did try a few different basses when I bought the Jazz but might not have experimented enough with neck sizes - think I looked at thinner necked basses thinking that they would be easier to play. As you said above, it is more of a dull ache than pain but it definitely slows your fretting down.
BTW, defo sounds dodgy that we may have compared hand sizes