Hamish,
My brother has the exact same problem.He has tried many different ways,and finds the only way he can get it straight ,is to sink the trailer.
His Keel rollers dont touch either,so there is no way to keep the keel straight.
His next move,is to fit a short length of 50X 50 gal tube,with stem clamps on it.so he can raise the keel rollers,and put some centreing rollers under the keel.
In his case,the wobble rollers pick up the strakes,and push the hull to one side.
David