Cylinder only hot at the top

[barrymmm]

Got a weird problem here. I changed a dual immersion recently as only the top of the cylinder was heating. I 'presumed' the bigger of the two elements in the immersion was busted. However, I got a call back to say the same problem was there.
I've checked all the wiring, measured the resistance across both elements and worked out that the immersion IS definitely working fine. From what I can tell, it seems that only the top of the cylinder is heating up and the thermostat is turning it off even when it's set to heat the full cylinder.
There's no leak on the system that's drawing the hot water off the top so I'm stuck.
I've considered wiring it up via an external thermostat on the cylinder. Has anyone else had this problem or suggest any other fix?

 

[kevplumb]

has it got the right stat/s in

dual element should have 2 if memory serves

 

[barrymmm]

Hey thanks for the quick response,
Yeah, there's the normal adjustable thermostat preset to 60 deg. and I've subsequently upped that to 70 deg. just to see if I could 'push' more heat down the cylinder but that hasn't done the trick. The safety stat is there too and hasn't been tripping out. It's got 'bath' and sink' connections and a common neutral that runs through the safety stat.
What's really puzzling me is that even when the large element is on only the top of the cylinder is heating up.

 

[kevplumb]

possibly full of lime sludge etc

prob why the last element popped its clogs

worth a look

 

[barrymmm]

I'm not sure if the other one was damaged at all now seeing as the same problem persists. Can't say I noticed any sludge on the old one when I took it out. There doesn't seem to be any discolouration of the water from the taps either so that's unlikely.
The quality of the water in the area is very good and lime is not a problem.
I just checked to see if the central heating pump was on because I figured there might be cold water circulating through the coil and pulling the heat away but that's not it either. I'm totally stumped...