Not that it will alter the Caster but what wheels and tyres / size are fitted to the car and how many miles have they done.

Are they matching tyres at all 4 corners?

All four tyres are like new (>1500 miles) Which magazine best buy premium Goodyear. About 4 weeks old. I think they are 15 inch with Skoda alloy wheels. Exact same size as the original tyres.

18 months or more is a long time since you first noticed it and you seem to have spent a lot in that time to try and resolve things, 

hopefully a well qualified and competent technician can find what is causing the issue and resolve that, just do not spend out on trial and error with anyone else that has not a clue really what the issue is.

Do you know of anywer nearby that has a mot ramp with the single man ramp? It has a plate that lets them shake the car with the full weight on it, they may be able to help you.