I popped to the shop and they made some adjustments but only using the barrel adjusters and it did seem ok on the ride back from the shop but I guess I will know after a few commutes but as I don't ride every day that will be later in the week. If it happens again I will need to be more forceful with the store.
It's helpful to have a basic understanding of how the gears work (the sort of stuff you pick up as you go along) to help diagnose common problems so here's my simple take on the rear derailleur (please feel free to correct any errors here, I'm not an authority on this!):
The large spring in the rear mech is in a constant state of pulling the chain toward the smallest sprocket. When you change gear/index down the range to a bigger sprocket, you are tightening the gear cable and it's pulling against the spring, moving the derailleur 'in' and pulling the chain across (toward the wheel hub) onto the next sprocket. The further you go down the gears (to the bigger sprockets) the more tension you are putting on the spring and the cable sometimes needs a bit more 'oomph' to get into the correct position; its a bit of a balancing act, cable vs. spring. Putting some extra tension on the cable can cure the problem of the chain not indexing correctly down the gears, but if you overtighten the cable you'll get the opposite effect and the chain will want to skip the other way, and at the extreme you could 'lose' a gear altogether at the top end (been there, got the t-shirt)
I tend to adjust the barrel a quarter of a turn at a time if I need to, that's all it takes.
No point getting into hi/lo limits here as once set these are generally ok, so don't change those adjusters unless you really need to.
for no logical reason.
