my wife has just been issued her settlement visa. she used a visa agent and they wrote 30/12/10 to be the day my wife would like to travel. in her passport the visa starts from 30/12/10 so i take it she can't fly before the 30th?

That's correct ( unless she flies on the 29th and arrives on the 30th). In any case, the airline won't allow her to check in as the visa is not valid for entry to UK unless she arrives on the 30th or after.