Indesit fridge/freezer model ban 12nf the fridge has stopped getting cold but the freezer is cold and the compressor is working its frost free but suspect it might be frozen and stopping the air flow to the fridge could this be the problem if not then what might cause this?

ok.. none of what you have done involved taking the back off.. if you can get to the compressor itself, you will find it has a dinky 'computer' style fan.. but invariably, some fool of a designer, runs wires very close, and uses low quality cable ties.. which break, causing the wires to spring up and jam the fan.. (especially if it has been moved recently) significantly reducing your compressors ability to chill your appliance.

get some decent cable ties, check and secure rogue wires, and things should sort themselves back to the right temperatures within a few hours.

Please add the make and model of your fridge. Is it frost free or not?

If there is no cooling at all , before we go further we must confirm the first step of confirming the compressor and gas pressure. So shut off the fridge and leave it off while you clean and check the evaporator for frost. If frost is seen it is a good indication as the compressor is working with sufficient gas within.
Now after about 6 hours plug in the fridge to start. Observe cooling to happen in about one hour and ice to from under three hours.
Earlier make sure that the thermostat was set to normal mode for freezer and fridge. If there is no ice formation we have a big issue as the compressor and gas must be checked out. If compressor is running check the pressure, get help to test gas pressure. If not you will need to fill up ,If gas pressure is good the possibility of the compressor being inefficient is high. In such a case you must take consensus to estimate and decide, if RPM of motor is low check capacitor.
However if Ice forms then observe the fridge for about 24 hours for cooling to continue and the fridge to cut off. If not the auto defrost timer, or the thermostat must be checked.

The ventilation fan that circulates the COLD air from the evaporator coils could get clogged with frost and so could be jammed. Also the wires to the fan can be worn out due to the frost. If so check if the defrost timer works to switch the heater to remove the frost. Also the check if the compressor is shut down by the timer /thermostat . If the heaters are not working, check continuity of the heaters.
So attempt to test using a multi-meter / clamp tester from the outside and make your observations so that we come to the solution.

Please check this link for tips to work.

http://www.espares.co.uk/advice/fridges-and-freezers/a/2

Please checck to see if your thermostat lines are in order or if some of the connecting wires are damaged bby frost. First of all switch off the fridge and allow all frost to clear. then check for the fan circuit, the auto defrost timer circuit. Now after few hours switch on the fridge. Check to see if the compressor is running the back side is getting warm and the freezer is cooling. If not check if the compressor is pumping well or being short of gas-- get help -. If not the timer could be the culprit as after the cooling takes place the fridge does not defrost. Check this out and the fault is confirmed if your fridge works fine now. Hope you try. Good day

If you take the rear cover off the inside of the freezer you can see the evaporator coils. observe the frost pattern. Light frost everywhere(NORMAL) or a partial pattern of ice(LOW ON FREON) or nothing(LOW FREON OR COMPRESSOR PROBLEM).
.

if it have one compressor the gas in the system are low . but if it have 2 compressors the thermostat can be the problem

the thermostat in the fridge is turning off your compressor before the freezer has completed its job.this is because it doesnt care about the freezer temperature only the temperature where its located in this case the fridge,try completely defrosting the fridge and restarting it

Check freezer... for Frost build up and if Fan is blowing, Check condenser... To make sure clean, and the fan is blowing (by compressor), Check recirclating Vents in refrigerator... to make sure theres no blockage, CONTROLS ...should be in the middle setting, YOU CAN TRY turning FREEZER CONTROL To A WARMER FREEZER SETTING Allowing.. MORE AIR.. to refrigertor side

if compressor is running, then you are correct in saying no point in replacing stat. by sounds of it you have a faulty compressor fault or a syatem fault - impossible to dignose by remotely. this is not a diy job i'm afraid

I would first check to see if the evaporator in the freezer is all iced up. If it is thats whay you do not have any cooloing. And If that is the case you have a defrost problem and it is not short on gas. If you see only a few coils on the evaporator frosted up then you do have a leak in the system and my be covered under the manufactures warrenty.