If that's £140 fitted then you're talking £35 a tyre. Even if he's supplied them at trade price and charged no labour for fitting that's very cheap! I'd be pointing towards the tyres being your issue and not any other part of the suspension. Take your car somewhere else and get some decent tyres fitted.
Could well be another case of 'buy cheap, buy twice'
Could well be another case of 'buy cheap, buy twice'