After someone PM'ed me re what was the eventual answer to the question. I though I should say what happened here for others to benefit by.
Replacing the seal is dead easy:
Remove the shift lever off the shaft.
Remove the 4 nuts retaining the shift cable mounting plate and the plate over the shaft seal
. Carefully pry or tap the plate to break the seal.
Replace he shaft seal
Reinstall all the bits in reverse order and make sure you seal the plate as it is reinstalled.
Voila. Took about 40 mins from start to end. It was done in the boat with the motor
in place. You could do it mid cruise
if you had to & I now carry spares just in case.