Dimplex CXLN tripped - how do I reset?

Hi there. I've got a Dimplex CXLN storage heater. I believe the heat input has tripped out.

Now I've taken the cover off and I know the area I'm looking for. However when I turn the input dial the heater is clicking but there doesn't seem to be any heat coming out. I can also see a silver tab which is sticking up a little bit, I've tried pushing it down with a screwdriver but its not sticking - what am I doing wrong???

Thanks for any help you can give