Under JCT Standard Building Contract 2009 is a contractor entitled to simply walk off site if an employer fails to pay?

The law is generally not sympathetic to such drastic action, even though its motive is understandable.

Under JCT Standard Building Contract 2009, the contractor is legally permitted to issue a 7-day written notice under clause 4.14 and then to suspend duties should it not receive satisfactory payment. Alternatively, a contractor can issue a default notice prior to termination. This is dealt with by clause 8.1.1.