AusBonsai

Hi everyone
just a question , how much can I cut the roots on this Mel the next time I repot

That will depend on which mel it is. There are so many and from what I can work out some tolerate pruning and root pruning better than others.
I generally cut up to 3/4 of the roots of all my trees, natives included.
I tend to go for broke and I'd cut above where they have knees (too long in the first tube before potting on ) but that won't leave much root and i can't guarantee it will survive.
If in doubt, cut a few of the roots short and leave the others long enough to sustain the tree while the shortened ones grow new feeder roots then cut the others the following year.

Thanks Shibui , I've got a year to think about it . At the moment i like the second option
Peterb