Only £50? that would just be the
sprag bearing itself then. Lucky you can buy that separately, on the Suzuki it's the whole unit at £235.
If he hasn't fitted the new
sprag bearing and you haven't paid for it then I would get him to sort the transfer gears first and check again, then leave him with the cost of the new bearing.
Then if I had any more issues I would take it to a different mechanic cause you would know straight away if it was the
sprag bearing that had gone.
Failing that I would then tell you to sell it to me for £350

with the new
sprag bearing