here's a fix that means no more messing around with allen keys.
Heres a very cheap fix.
Buy two ordinary hex head bolts with the same thread as the Triumph ones. Thread them through the lugs on the frame but from behind i.e the other way, so that you end up with two threaded studs protruding from where the seat bolts normally fit. You might have to drop the mudguard and bend the frame tab slightly ( I did) Then get two wing nuts. Put the seat on threading the seat tabs over the studs. Fix the seat on with the wing nuts. Much cheaper than fancy over priced extender bolts. Works just as well as you can now remove the seat easily with your hands.
Here's some pics to explain. Attached Thumbnails