thanks for all the suggestions guys

The enlarger is fine - good solid double glass carrier with spirit level checks of baseboard, carrier and lens plane before each session. Old negatives are giving me good sharp prints

mikewhi ... sl66 backs are adaptable between 120 and 220 at the flick of a switch -i cant use 120 at the 220 setting, so I dont think thats it.

Ralph - The film holders look fine ... I'm getting the problem using both of my two backs so unless they both have the same alignment issues ....?

I'm reliably informed that both the focusing screen and film plane alignments have been checked with no faults showing ... so???

what a nightmare!