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GOT IT!

Thanks both of you.

After getting the 'tweak' out of the chassis, once installed in the body it was back, as bad as ever. Initially I tried to set up 'stagger' on the body posts - a 1mm O-ring on one side between body and chassis

BUT .... 

I finally noticed the central metal grill was sticking down below the body line and acting as a pivot point for the chassis which, as I habitually do the screws up in the same order, was pulling the chassis higher into the body on one side of the front than the other. 

Ground off the offending protuberance so the body and chassis can sit square to one another and she pulls up in a straight line!

Now I guess I need to check the others, just in case the grill is imparting a twist into the other 'non-straight line' brakers.

P.S. Removing the stress point the grill was creating has also helped the gear alignment so now she's sweet as a nut.
P.P.S. The twist imparted to the chassis stayed with it when tested on it's own earlier - but it's already 'relaxing' into its naturally flat state left to its own devices.