'Left' button on DSi has stopped working - any ideas on getting it to work pls

Hi, the 'left' button (on the back face of the device - same face as the cartridge slot) on one of my boy's DSi has stopped working and he needs it for some games, especially Diddy Kong Racing.

I have read several items on the 'net' where they say that blowing into the sides of the button, gets it working. I've done this and it actually worked for a few minutes but then stopped. I also tried blowing it with a tin of air but to no avail.

I guess the blowing works because the contacts have filled with dust?

Is there any other way of getting the button to work? Can the buttons be got to work by opening the case anyhow?

Any answers/suggestions appreciated.

BTW the DSi is 3yrs old and has been used.

Many thanks