kind of writing this up a fair way into the project - I have been happy with bench tests but limited to a supply of 2 A which is a long way from the real world.
So to move to the next step i needed a much bigger supply something at around 600W at about 20V - (which is the open circuit voltage of the solar panels.
hmm 2 x 12V batteries in series is only 24 close enough.
So I wired up 2 100AH 12 volt batteries in series and connected to the solar input of the controller but to control things a little I put a 60W car headlight globe in series with the batteries to limit the current to about 5A or so, hooked up a battery to charge and all went well.
With minimal heat sinking on the FET no noticeable temperature rise and things appeared to be going well so I decided to up the game a little and bypassed the globe.
This is where things went wrong very quickly the battery supply was quiet happy to deliver in excess of 100A (the maximum reading on the current sensor ) before the FET went crack and removed its face - I really should have had something in place to limit the current to a more realistic 50A - given I have a 24V supply and need to drop to 14V.
24 - 14 = 10V
10/50 = 0.2 Ohm resistor capable of handling 500W should do the trick.
alternatively if i replace one of the 12V batteries with a 6V that means I only need to drop from 18V to 14V.
18 - 14 = 4V
4/50 = .08Ohms
looks like 11m of 2.5mm copper wire will do the job.