What does he mean by 'checked the tyres'? Uneven tyre wear can cause this problem, you can check simply by looking them, if you put a screwdriver in each section of the tread, or a coin would do, slide you fingers down until they touch the surface of the tyre and repeat this across the width and see if the tread depth is equal across the tyre. You could try swapping the front and back tyres.

If there is a fault on the car it could be that the camber angles are incorrect, again this is easy to check, park the car on a level surface and put a spirit level vertically across the outside of the front tyres, they should lean very slightly inwards at the bottom, the ammount doesn't matter as much asit being the same on both sides.

Frustrated enough to consider getting rid, despite the loss. Is there any likely cause that they could have missed?

Bet it's running Bridgestone Turanza tyres? They tramline like no other tyre when they get low on tread.

I'm betting changing the front tyres will totally cure the problem. My Mondeo was the same on worn Bridgestone Turanza ER300 - replaced them with Pirelli P7 and it now drives sweet as a nut.