Neil,
Sorry, I should have clarified but the topic of transmission lube has been covered before and I forget that not everyone has seen the old posts. The method is to drag you finger through it to make a groove and see how long it take to fill in. Don't worry about it filling in perfectly without a trace.
The danger of getting it wrong is thus: Too thick, the gears cut slots through the grease and push it all aside and it never flows back in to lubricate the gears (or not fast enough). Worse, it never gets in to lubricate the main shaft where it runs through the sleeve gear. Too thin, it leaks through the sleeve gear where the main shaft passes through. However it is a British motorcycle and it is expected to leak oil...
-Doug