Fan running intermittently at full speed

Hello.

The fan on my MacBook recently started running high. The Mac (2.4 GHz Intel Core 2 Duo, Memory 2 GB 1067 MHz DDR3) was purchased in September 2010, and was returned to Apple free of charge last year for a new case, keyboard and rubber base as the originals were faulty and had cracked. Upon encountering the fan issue I consulted other discussions on this website and followed the advice to conduct a SMC reset and fully update all software (running OS X 10.8.5 (12F45) as of 30th Dec 2013).

When this failed to have any effect I booked an appointment at the Apple Store in Brighton, where an MRI test was conducted highlighting a number of hardware errors. The shop assistant, who was very polite and helpful, inspected the inside of the machine and reseated a cable inside, then conducted a re-test and declared the problem fixed. Unfortunately, as soon as I switched the computer on at home the fan was revving full blast again.

On my second trip to the Apple store (today), the ironically named Genius (who I suspect may not be a Mensa member) who greeted me conducted the same test, and, after looking up what the results meant on his IPad, suggested that the 'Temp Sensor MCP Die Top Side (internal)' is not reading any temperature. On the notes which I asked him to print out out he has written that 'take apart suggests this sensor is part of the logic board', although at no point did he dismantle the machine to verify this. At this point he casually mentioned that a repair would require a new logic board and this would set me back around £400. The machine cost me £850.

When I requested that he perhaps try some further investigation to rule out the possibility of, for example, a loose cable connection, he replied that it would certainly require a new MLB and there was no point taking it apart like his colleague had done. He then explained that he knew what was wrong with peoples' computers before they had even told him.

My questions are:

Is there any possibility of the cause of the problem being something minor, and therefore cheaper to repair, and;

Am I fully responsible for the repair cost of this component? The case and keyboard problems were recognised by Apple as faulty and were repaired for free as a consequence. As the machine has been very well looked after and the component in question has failed through no fault of mine, should Apple recognise this as a fault, and if so, how do I go about requesting this? I really can't justify spending £1300 in total on what is a fairly basic, and reasonably modern computer.

As I write this the fan is working perfectly normally and the MacBook is behaving like new. Sorry for the rant, I appreciate your help! Thanks in frustration.

Jim.