Residency visas

Hi,
I'm in a new pickle now.
An emergency has happened and we will need to fly back to UK for a week.
My wife has a new passport but we haven't yet transferred the residency.
Can she leave on the new passport (with no residency visa in it) and then return on the old passport with the residency in that one?
I know the return is OK as my little son did it last year, but it's the going out that I don't know about.
Thanks

I can answer my own question!!!
I went to the airport to ask and they said that you cannot leave the country until your visa has been transferred across into your new passport.