Bloody hell that is expensive! For that price you could have got some Eibach and had them fitted. VX springs are prone to issues.
Did the bloke doing the work give you an invoice or anything for the parts he purchased? If he got them from VX, it should have a part number on it to at last see if the correct springs have been fitted.
It's certainly strange how you have described it happening, and given it's intermittent, it's hard to really take a stab at what could be happening.
How long into the drive does the sound then go away? Or does it go away once you turn as you have described?