Finally managed to get the car into a garage. It seems the rattle is coming from the steering rack itself, and could only be found once on a ramp and with two mechanics shaking things vigorously. I've been advised that the play is minor and at no risk of breaking/snapping anything so I have time to source a replacement or consider my options. I do less than 3000 miles a year so it was suggested that it may be cheaper to live with the rattle and the extra wear on the tyres.
