Hi James.
I'm not familair with any make or model, however, from what I've learnt, there needs to be flow for the heaters to work.
So these things have flow switches.
Assuming you have a good flow, then i'd check the following.
If your'e competent with a multi-meter, i'd start by removing the power.
Find the heater element, remove 1 wire and check for continuity across the terminals.
(remove 1 wire, just to stop you getting a reading through the pcb)
As it's basically a giant resistor, i would expect to see some measurements from it.
I'm presuming it's a 2kw heater, so provided my old school formulas still work, you should see about 25 ohms resistance through it.
After this i'd maybe look at the flow switch. It may look something like this
http://www.poolandspacentre.co.uk/balboa-pressure-switch.html?gclid=COeOqLGKtLgCFafLtAodmS4A4AWith the pumps running, i'd expect either a closed circuit, and the opposite with no pumps running.
I dare say someone will be along shortly to advise better.