Having initially failed to work out how the handbrake adjustors worked I did some research on the Internet and on the RHOCar forum and figured it out. So up came the back wheels again.
The basic problem was the brake did not begin to bite until the 4th-5th click of the ratchet and wasn't fully on until the 7th-8th click.
So I wound it up (The plastic unit just sits in the chassis member and when you rotate it it moves along the outer cable, taking up the slack) until the bite started at the 2nd-3rd click and it was full on by 4-5th click. Much better now.
Here is a picture of the unit just before I tighten up the second (nearest) locknut.
Still don't know the purpose of the extra washers ?