My 250R is stuck in sixth. It had occasionaly done this but cleared itself. After an incident the other day it now refuses to budge from sixth. Any advice would be welcome before I split it.

 

you're going to have to split the cases.it sounds like you have a bent shift fork.

 

This happened to me on my old 250R but it was 3rd gear it was stuck in. When I split my cases all the dogs were gone on the 3/4 gearset.
While you have it apart get new shift forks & look the shift drum over good.