Why does my car have an intermittent judder?

Two weeks ago I had the front driver tyre changed and ever since I've had an intermittent judder @ 70mph. I think that the tyre change is a red herring as I have subsequently had all my wheels balanced, the car has been tracked and all tyres have been swapped around to establish if any of them were faulty. I reckon that the judder is a mechanical failing but what could be the cause of it, will it be expensive to rectify and could it be dangerous? It is a 55 diesel fiesta.