Thanks "makemineadouble" (me too) thanks for the advise, this is an older model, I have been re looking at the problem and decided to reduce the flanges on the drivers and "Hey Presto" it has cured the problem although it is still not an easy rider, I found one set of drivers was 10thou larger than the rear wheels at the flanges, enough to rock the chassis when passing over the frogs.
I like your moniker, we have a saying here in Canada when we order from our favorite coffee shop 'Double Double' meaning double cream double sugar. It became so popular it has been put in the Canadian Dictionary. Anyway thanks for your input