This study aims to evaluate the efficacy of a novel physiotherapy intervention incorporating the newest and most promising form of CBT, Acceptance and Commitment Therapy (ACT) against usual physiotherapy care. The investigators believe this will optimise outcome for patients, the NHS and society at large. The investigators have conducted a proof of concept feasibility study to develop a brief physiotherapist-delivered ACT-based intervention, 'PACT', consisting of two one hour sessions plus a follow-up phone call. The aim is to test PACT against usual physiotherapy care in a phase 2 trial. The main research question is whether PACT (physiotherapist delivered ACT-based treatment) is effective. The investigators hypothesise that the group receiving PACT will have improved function at 3 months compared to the treatment as usual group, and that this will be maintained at 12 months. The primary objective of this study is to evaluate the efficacy of PACT, a novel brief ACT-based physiotherapy intervention for chronic low back pain, at the primary end point of function at 3 months follow-up.