You never mentioned they'd seen a dark 4x4 in your original post, just that they'd driven up and down the lane several times? Have LE not been back in touch with your parents at all? Were they able to take a reg number?
They had (my dad in particular as he is a member at Long Ashton Golf Club) driven down the lane several times. My mum was in the car the evening of Christmas Eve and remarked at it being a stupid place to park a car. When Joanna was found on Christmas morning they discussed in detail that they must have driven past her body and my mum told me about the dark 4 x 4.
However, at the time it was thought that the body had been in situ for a number of days and then CJ was arrested so they didn't give much thought to it.
After CJ was released and a statement by LE was made stating they didn't know when the body had been dumped my parents rang. They have not received a call back, although by then they couldn't remember exactly where the 4 x 4 had been and told LE this (I can remember them talking about it in detail to me on Christmas Day though).
I'm just a little surprised that a week or so later they were stopping drivers on the lane to question them but never contacted my parents.
No they didn't take a registration number, they just thought it was a stupid place to park and Joanna had been missing a week by then. They obviously didn't expect her body to be found there the following day.