As we have a large cavity (about 180mm), standard wall ties would not be long enough.  We are using Ancon 2 part stainless steel ties.  These come in two parts, which clip together.  The advantage of having them in two parts is that you are not left with a huge tie hanging out of the wall after you have built up one leaf and are building up the other leaf.  It also makes them slightly easier to position because the bit hanging out of the masonry is not as heavy.