My heater runs for about 10 minutes then the flame cuts out and can't be turned on again till it cools down for half an hour...

sounds like running out of gas pressure

Apparently the photocell needs to see a flame of the correct colour or it will shut down so the nozzle and spinner has to be in very good condition - diesel might be clean but is the inside of the tank clean?
The fit between the spinner and nozzle is so very close it can stick with very small particles.

Can't speak for the Reddy Heater but a bad spark transformer seems a very common problem with that type of heater - ensure the spark gap is correct.

check the tank for any blockage and check outlet pipe .

So the unit made a noise 10 minutes after it turned off. Then again when it wasn't running??? Maybe it's not the a.c. But, if refrigerant was added did they try to find a leak?

If you need further help, reach me via phone at https://www.6ya.com/expert/thomas_092728000e6acb79

I had the identical problem with the same model. Technician eventually diagnosed faulty fan; the fan cools the heat exchanger. When fan fails, exchanger overheats and boiler automatically shuts down. It is possible to restart the boiler when it cools down but when the heat rises again - off it goes! Only answer is to replace the fan. My technician did the same as yours - on first visit he got the boiler running and left. Half hour later, boiler shut down. I read the installation manual and operating instructions and when he called again I pointed him in direction of fan, which had failed completely. These guys!

I have just had that problem. Repairer said it was a chip in the power supply. Just got the set back this morning, ok for about 6 hours then cut out. Felt quite hot on LHS Will switch on again after a while, then cut out again. Suspect overheating problem took out PSU chip. Chip replaced but original fault still present

FL 2 is ignition failure,
If it ignites as you and the heat icon comes on, then it quits it sounds like you are losing gas pressure, no gas flow or it is to low, or air is in the gas line, if there is ignition then your flame sensor is not sensing the flame. or it needs to be replaced.

Sounds like you have a wick that has burnt unevenly in the past and now there is not a good setting for an optimum burn. Either the wick needs to be trimmed even or you need a new wick.

I wrote this for another person with a heater problem and thought it may help you as well.

Before you go changing parts, clean up the pilot assembly. When you heat metal and let it cool and heat it again it causes it to corrode. Also for some reason these areas seem to attract little spiders who make their homes here. This causes gas flow problems which in turn could affect the size and direction the pilot flame. For if it does not touch the thermocouple, thermopile or whatever that little thingy is called that tells the gas valve to allow the gas to flow...it will not tell the gas valvle to let it flow. Remove the corrosion, spiders, spider webs and any other insects then sand the thermocouple, thermopile or whatever that little thingy is with a rough sandpaper to remove corrosion. It will likely work after that.

I had them do the same thing. The gas flow problem caused a week pilot. Any slight breese pulled the pilot off of the thermocouple and out it went.
Pull the gas line off of the pilot and make sure you have good gas pressure when you press the pilot button. If you feel, hear and smell it it is good gas pressure.