It would be if I'd had a Time Machine back up (only just got a Mac!)- the ironic thing is I've just switched over to Mac from Windows two weeks ago after being endlessly told how much better Macs were for music, hence the assumption that nothing could go wrong! It has....
I agree I should've checked but thanks for the advice - seems a little complicated to revert back to Yosemite, but I'll have another look.