I think there's a few older members past and present who visited Peakirk, but you may be right about Cardiff Zoo (though some have visited the collection in its later incarnation as Welsh Hawking Centre).
Been to Cardiff, it was called Barry Zoo I think? They had the chimp Melody from Dudley that later ended up at Twycross.
How about Ferndown(Dorset) or Bideford( Devon) zoos?