My STI has started pulling right under acceleration which is not good. Have had the tracking all done and they say nothing loose. Seems could be mechanical ? Any ideas?

Have the rear wheels taken a knock? That would mess the thurst angle up. Could be a few things I.e. Collapsed bushes or suspension, brakes, anything drive line related really. 

Brakes in a straight line. Fitters checkedsuspension and bushes on front and could not find any movement. Dont think rear wheels have taken a knock but who knows with all the pot holes.

Wheel bearings all good? 

And it's more likely a problem with the rear as that's where the power goes. 

You could try getting a 4 wheel alignment done, as this would show you if your camber or Caistor is out .As tracking only checks 1 axis on just the front wheels .

If it's not broke .....upgrade it 😊

So had the tracking done and the car is worse. Wife drove it and stated it feels loose and titchy.

So 4 wheel alignment done and ok but cant adjust front caster as bolts stuck so need to get garage to lossen them. 

Seems the new tyre might be problem as tyre same make and model but new version of compound and full tread could be effecting the handling .told thats why you should change tyres in pairs. Ie i should spend £150 to replace a good tyre to see if it sorts issue.