Driving to work this morning (about 15 miles on the motorway) At about 60mph I noticed the steering wheel starting to shake very subtly about a cm back and forth .. The steering itself is still very smooth, stable and responsive..

It's not really noticeable at low speeds, starts to become obvious at 60mph, gets worse around 70mph and then begins dropping off around 80mph, then feels completely smooth again at 90mph (just testing of course..).

I've had the front tyres changed 3 days a go, will it be something to do with that? incorrect size, pressure, mounted incorrectly? (if thats even possible), damaged during mount?

I'm pretty clueless with cars so would just like to know where I should look first.

I could be imagining things but fairly certain it wasn't there last week before the tyres where changed, and this is the first time i've drove at 60+ long enough to notice it.

Any ideas?

you need the wheel balancing done take it back to the place that fitted the new tyres 

Sounds like incorrectly balanced wheels, or maybe you have lost a balance weight, as said above take it back to the tyre fitters.

Youll have worn down the "high spots" on the new tyres and now the tyres will be unbalanced again. Just take it back and have them re-balance them and you should be right as rain.

err as i said