Are Spares Available for the MysonApollo 50si?

The boiler is clicking off and re-starting when it has been running a while. BGas engineer says it does this when it gets too hot. Apparently spares are not available he says & he can;t get the gizmos to fix the boiler. Says I need a new boiler, offered some expensive things he thought might solve the problem like flushing the system & something else - (thousand pds plus). If I switch the boiler off it will work for a while before cutting off. Please advise if spares available & is it worth repairing. Many thanks.