Thanks guys.
I've used shims on the shafts that joins the caster blocks and the lower arms, so they can't move forwards or backwords on the shaft, but the play seems to be due to the holes in the lower arms are a little to big, so the shafts can move a little around in the holes. It's not much, but it's very noticable when you "amplify" it out to the edges of the wheels (I'm not sure that sentence made much sense, but it's the best I could do
)
Cheers
\Martin