DS had single jabs for Mumps, Measles and Rubella, and is now overdue the second round. I've just had a letter stating "for those waiting for Mumps immunisations we expect there to be a Jeryll Lyn Mumps available for the UK by the beginning of next year".
So it does sound like there might be something on the way. We're going to go ahead with single Measles for now, and see if a Mumps vaccine does become available.