Sorry for not following up on this patient. I was recently on holidays. I ended up finding some instability of L4 on L5 with clinical testing and sent the patient to his doctor to order an xray. It turns out that the patient had a Wikipedia reference-linkspondylolisthesis of L4 on L5 (grade 2). I focussed treatment on core stability (neutral and flexion exercises) as well as mobilising his stiff R hip to take some of the strain off the low back.
He is now asymptomatic. The thing I found tricky was that he had no real dural signs despite the listhesis. His dislike for extension makes sense due to his listhesis. Thanks for all your feedback.

Willeseden