Although the original poster is probably long gone, I'd just like to report that I fixed this irritating airbag fault code this morning.
I had removed both front seats - that produced fault codes for side airbags not present, obviously enough. But after replacing the seats, I had fault code 21 for driver's curtain airbag, "value too high".
The solution lies in the boot.
Behind the carpet trim, right hand side, there are two plugs for yellow airbag cables. I tried unplugging the yellow plug (as opposed to the orange plug) and I had fault code 21 twice, but this time, "not present" as well as "value too high".
After plugging back in, I was able to clear both fault code 21s without either returning.
I took the trouble to attach the plugs properly to their little black plastic cover plate, which allows you to reach the plugs without the need to remove the carpet. See picture.