Baxi Hot Water not firing up?

Hi All,
I have a Baxi Combi 80e. CH works fine, but the HW won't fire up unless the CH is on. I've read through these posts that suggest the h/e plate might be blocked. I've just removed this and can't see any obvious problems - having not seen one before I've nothing to compare it to. What/where would be blocked? on the h/e or the pipes. It certainly let out some water when I drained it!

I've put it all back together and it hasn't fixed the problem. Any other areas that could be causing this problem?

I didnt use any de scaling agent. I simply took out the h/e gave it a shake and a blow and put it back. there wasn't any visible corrosion or anything like that on the outer casing. How can I tell if it's blocked?