Have put this off long enough and with the advent of 10 speed mtb drivechains thought it only right to remain a decade behind and go from 8 to 9 speed. Biting the bullet I have invested in new shifters, chain and cassette but the shifters are the Deore SL-M590 two way shift jobs (2nd hand off here of course) but will they be compatible with my new older pre-shadow style XT rear mech, do they still pull the same ratio or with Shadow rear mechs etc, have Shimano gone and changed their pull ratio that will require a new rear mech? Hope this makes sense …

Thanks
Luddite in training

they'll work with any 9 speed mechs.[Edit – well Shimano ones]

I see you've found the internets Grandad, well done 🙂

Less of the "Grandad" sonny, and just how many internets are there exactly?

How many Itnernets?

http://en.wikipedia.org/wiki/Deep_Web

depends who you ask.

My view is that we all have our own one…

Your 8 spd chainrings will be compatible, but I'd like to bet your new chain will skip like mad on them. Middle rings often need replacing, granny rings can often be filed to remove burrs, outer rings usually cope fine.