Baxi Bahama flowswitch replaced but safety valve now runs constantly? ?

Have a baxi bahama, the flow switch was replaced yesterday by plumber but after he left the safety overflow pipe ran constantly. He came and said I needed a knew safety valve which he replaced this morning. He left and the safety overflow has started to run constantly again. The pressure guage has never been above 1 - 1 and a 1/2 bar. Any ideas what could be the problem?