Rubys nursery is tiny box room so we didnt have space for changing unit..we got a cot top changer for about £20 in Mothercare, it has bult up sides to stop her rolling off into cot (although at the mo she squirms and tries to roll into cot, have to change her with one hand pinning her legs down a the mo!!!) It has been brillant, highly recommended.
We have a changing mat downstairs which we use for the occasional change on the pouffee, but to be onest its easier to take her upstairs.
The changer comes off and stores behind her door.