Well.....they've been out about 5 times now, first of all replaced pump, then diverter valve, and at time said heat exchanger had some muck in it, but that they had got some/ most of it out. THey then had to come back as a something was leaking, and water temp was nowhere near high enough. At this stage they said 'could be the thermister, or could be the heat exchanger - we'll change them both'. When they came today I was expecting the thermister and the heat exchanger to be replaced, only to be told they were only replacing the thermister because home care doesnt cover sludge problems.
THey never mentioned sludge problem at first visit (only ~2.5 months ago), and never told me I should get it sorted myself outside of homecare agreement, before today. THey also said that my job was a 'monitored job' cos of number of times they'd been out - although at least 2 of those visits were because they'd rushed the previous visist so much they'd not done something correctly (at one stage they went away without reconnecting a thermister cable to the diverter valve so water was superheating at that point.....)...
Reckon I should insist on them doing it for free? THey said it'd cost £120.