Can't drop into the 2 smallest cogs

[Globalti]

The only thing that has changed is that, as the OP wrote, the bike has been laid up for a while. This is at the time when roads are wet and salty.

The steel reinforcements in the cable outer are rusty and manky. Oiling the cable won't restore proper performance, it needs a new cable outer at the rear derailleur. A five minute job.

 

[Sterlo]

Didn't know I'd open up a can of worms here, it was only a simple ask. I undid the cable from the deraileur, gave it a bit of a wiggle to make sure it was free (seems to be okay) and reattached it, everything is great now, better than ever in fact. I don't think it was ever right from new to be honest, I suppose that's the issue with buying online (sorry to all of the LBS enthusiasts, if you want me to leave now, just let me know!).

 

[fossyant]

As said, new cables. If they are dragging a bit then the mech won't change well, especially at the bottom end as spring tension is much less.