Thanks all. I remember in about 1987/88 looking through a Thomson summer brochure and virtually all the Bristol and Cardiff flights were to be operated by 'YM' - Airways Cymru - having been Britannia hitherto. Similarly, all the Spanish flights to and from Leeds Bradford were handed to Hispania and their new 733s and 757s.
I too remember the 'Welsh Air' callsign.
I wish I could remember where I had read about BAZI's alleged dodgy serviceability record.