Brian, I've got the same hardware as you, but when I sent the command my Sky HD box doesn't react, although the light blinks on the Sky/RS232 interface.

I'm wondering if I'm keying in the RS232 commands correctly. e.g. for the 'Sky' button command I've tried using the ASCI code:
0x31 0x0C 0x80

I've also tried this as a HEX code:
310C80

But neither of these approches works. What format are you using for the code?