Hi Dave, and welcome!
I've not heard of that trick being used with a GB mainshaft before but no doubt someone will be along with more knowledge on that.
If that doesn't work it is possible to strip/rebuild the box without removing it from the bike first.
You might want to look at this thread with good info on reclaiming threads.viewtopic.php?f=4&t=8831
(PS be very careful about criticising Mr Ariel's fine chaincases around here.....!! )
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)