Myson Apollo 50B will not switch off?

Problem started when suppressor blew. Replaced suppressor but boiler then over-heated. Instructions pointed to faulty hi thermostat. Exchanged thermostat but still didn't respond. Boiler would work ok on low setting which I used over weekend whilst I got another stat. It has now stopped responding to lo thermostat although it will burn ok. I just daren't use it for fear of over-heating.