Ben Willcox wrote:

I've determined that the joist sizes I need are 2.4m long, 43mm thick,
and 170mm tall, i.e. not a standard size. (This is about 1 or 2 mm
undersized so I can be sure they'll fit - don't know whether I need to
take shrinkage/expansion into account here?)
However ringing round the local timber merchants, one has said that you
can't plane pressure treated joists as you'll lose the weather
resistance, and one has said they can saw it but not plane it (for the
same reason)!!??

Go to Jewsons and get them to cut the timber to whatever size you want
and treat it for you.