Fettling issues - slow shifting rear gears while riding I've recently had an issue with my gearing, struggling to shift down onto the last few cogs on the cassette. 

I've got a Shimano Ultegra 6800 groupset, but when I shift whilst riding, I get a "tick-tick-tick" then it shifts.

This isn't the issue though.
I understand that a tweak of the barrel adjuster should sort it.

My issue is that when I put my bike in the bike stand, it doesn't happen. 

I have thoroughly cleaned the bike and groupset, and re-lubed it. When shifting up and down to test, everything flies smoothly!

I then took it out on a test spin to make sure everything was hunky-dory, and my shifting issue was back!

Does anyone know of a way that I can replicate the riding conditions/forces on my bike whilst on the stand?
You need a local test route for this and other things. For some things just up and down your street will do, but for this issue you might need to put the bike through its paces properly. 

Ride your test route, paying particular attention to the feel of the problem. Stop the bike, tweak something relevant (your best guess) and ride it again. Better or worse? If better but not perfect - adjust some more the same way, if worse go the other way. 

You may be able to use your commute in the same way if commuting on this bike is an option. 

In your case I'd get it perfect on the stand, then tighten the rear derailleur cable a little. Test ride and see. 

Actually of course this is a tedious version of normal adjustment procedure. The nice thing about a rear derailleur adjustment issue is you don't need the bike on the stand for minor tweaks.