Year and week, adding zero before week number

Hi

I'm sure this is really simple!

If I use the following formula to display year and week gleaned from 2 other columns:

=A1&" "&B1

where A1 is 2001 and B1 is 01, I am getting 2001 1, but I want 2001 01!!
I've tried format/custom 0000 00 but it's not working for me..

I need to have a zero prefixed before any single digit weeks- how can i do this?
BTW I'm using excel 2007

thanks
Randalino