Been advised by my garage  to renew the bushes on the front lower suspension arm trailing and the rear trailing arms   any idea of cost   the car is 11 year + old   is it worth changing the front lower wishbone arms

You can change just do the bushes  on front wishbones but can be more cost effective for a garage to replace the complete wishbones due to labour costs. Expect to pay approx £120 for complete wishbones each and remember get the wheel alignement done.

Rear trailing arm bushes are a nightmare on the MK2 Focus. Leave them well alone unless they have excess movement. A little bit of cracking doesn't warrant changing the rears. 

All MK2 Focuses tend to have rotten rear suspensions, bolts snap or tear other bushes, then you need to replace complete arms. The upper trailing arm bush is known to cause the whole arm to bend trying to press the old bush out!

May as well change the fronts, they're really common and a much simpler fix