Gaz,i am sure there are more knowledgeable people here and i may be wrong but i have run a turbo with play in the shaft and i have never found it make a difference.You always get some play,not a lot but always some.Ive had my series 2 since new and it has now covered nearly 100,000 miles.just changed the turbo only coz the oil seals went but it still boosted well.dunno what a cossie produces boost wise though.I would be tempted to check elsewhere first,ignition and fuel etc.