Whats wrong with your power steering - is it a street Ka? Is it noisy? Does it feel like you reach a point of sudden resistance when turning lock to lock?
Dagenham have been good with my parents who bought a new Ka there.
I would completely recommend Simon at Halfway Garage on the Gravesend road where the car dealership is (about half way from Higham to Gravesend funnily enough) If theres anything i cant do i will take it to simon, he will always try and fit you in even if he is busy, my family have used him for years and years.
A bit closer to you is CE autos over by the Harrow pub near Bredhurst - Carl there has done a few bits for me in the past (including swapping some bits on a steering rack) and i cant argue with prices, also his uncle Mick owns the bodyshop opposite and he has done some spraying for me.
MOT's i get done at the place at the bottom of cooling road in Strood, nice and quick and honest as they dont do repairs.