Found this on http://www.pentaxforums.com/forums/161-ricoh-gr/261769-disassembling-gr-sensor-cleaning.html by forum member i83N. Hats off to him, all credit goes to him, i'm merely spreading the love.
This is for those of you who wants to do it yourself rather than sending it to service centres.
Here's a screenshot of the forum's thread:-
Just like what a lifeguard would say, "Swim at your own risk", or better yet, "Do it at your own risk, Warranty surely voided"
Here are the steps, good luck my young jedi....
By default, remove batteries and SD card.
1) First you must unscrew 5 screws. Begin with first hidden under grip skin screw.
..........then the other 4.
2) This will release the back panel. There are small ribbon connecting buttons, it pops out easily.
3) After removing back panel, unscrew 2 screws holding the LCD screen
4) Carefully unclip black and red wires around LCD. I think it's power for the left mic.
5) Unscrew 3 screws which are holding the sensor.
6) Clean sensor with microfiber cloth and Giottos Rocket Air Blower.
7) Assemble in backwards order!
Good luck guys, i've never done this, all credits to i83N.