hi kev
it is definately not advisable to bridge the gap where the 2 seperate substrates meet unfortunately mate...
there should be an expansion joint according to british standards. your ditra should stop at the point of the expansion joint and then continue on the other side (definately not bridge.. expansion has a much greater force of uncomprimised energy at the point of movement then deflection.
2mm is not enough of a gap between the concrete and the 22mm ply imo because most expansion gaps rely on 6-11mm of rubber and a couple of mil of trim either side. when you fit any timber flooring, the perimeter gap should be 10mm ie where it meets walls or in this case the concrete floor. 2mm is fine between boards when laying because the 2mm gaps accumulate to a larger amount over distance to allow for deflection..
hope this makes a bit of sense kev, recap dont bridge the gap with ditra and increase the join between the 2 substrates to accomodate an expansion joints,,
regards
Ed