Have a look at the link in this thread re the guy changing a Matrix in an hour.  Most garages will do it for  a decent cost as long as its not a VW or Skoda one 

thanks - I thought I'd read the entire thread so just went back but can't find the link you mention.  Sorry I still find the site tricky to navigate! (As I read the posts again the coolant warning light coming on was mentioned a few times.  I've just realised that this is the case with my Skoda.  It came on about 7 months ago.  Topped it up, all seemed fine.  Then it came on about a month ago.  Topped it up, 2 weeks later heating isn't working....   I guess what I want to try and do is rule out something more simple like a fuze or air lock.  Just before the heating went I plugged in a trailer electrics - that a bulb blew and the heating stopped working.  Is that a co-incidence?