They need 7 class 150s to cover 6 diagrams; the LM loan is to make the fleet up to 7 for this reason. Should there be two 150s unavailable for any reason, a 165 will be used as a last resort; it will only need narrower stepboards fitted in order to operate on that line.
Incidentally, it was supposed to be 150 126 on loan, but for some reason 150 132 came along instead.