Fitted some different wheels and now my discs rub. Adding shims just makes things worse as it's the 'other' side. Tried the 'loosen off bolts, pump, hold tighten'. The disc is just too close to the outer (left) pad with the piston quite far retracted. Is there anything else I can do? Is there something I can do to the hub move the disc towards the centre of the wheel?

If it's not being caused by one piston/pad sitting out further than the other, then you can get the fork/frame faced back, it's worth doing in the interests of allowing the pads to hit the disc square on anyway.

So even with no shims your rotor is hitting the outer (LHS) edge of the capier?
In which case your best option is to get the mounts faced.
Or
Put a washer between the hub and the fork temporarily.
then go for the facing option.

Thanks – didn't know you could get forks faced.