2 points:
the boiler is heating up and the boilerstat does appear to be doing its job, so get that checked as to why it is occuring
also the system is dirty and needs to be cleaned.before putting inhibitor in the tank drain and take off each rad that needs to be hosed out in your garden . this is a mundane task but if the system is dirty with sludge the inhibitor is a waste of time and money untill it is cleaned out.
also repeat your enquiry on www.fixeditforfree.com