I know this is an old thread but there is some relevant information.
I have had all my linkages replaced, gearbox oil replaced, bush and seals on front linkage replace and think adjustment is needed as I too have problems getting into 5th gear.
I was assuming it'll be the length of the "Gear linkage Ideal Rod" as it's adjustable, but this post suggests it's the one at the front.
I currently have battery, radiator, starter, carbs out of engine bay for refurb so an ideal time to adjust the linkage as full access.
Is it as simple as trying to select a gear, and then adjust front linkage 1/4 turn, and repeat? Or should I be double checking the Ideal Rod first?