Once again, thanks everyone for the many replies

I bought the bandsaw to do metal - the minimum blade speed is actually a trifle fast for that, although I may try a TCT blade, as I've had great results with TCT chop saw blades in the mill.
I think the main problem is blade tension - Andrew, yes, I have a blade tension guide & it's tensioned to the appropriate blade width, however, I've seen videos now which suggest that these are not necessarily particularly accurate, especially if your blade length isn't the exact manufacturer specified length, and you're supposed to tension it until just before the weld snaps.... quite how one does that without destroying the blade, I'm not entirely sure!
Also, the guides aren't set to within a gnat's of the blade width, so that must be allowing some movement. The backstop's OK.
Next time I'm going to cut anything big, I'll spend some time getting it configured. In the meantime, for slicing thin sheet ali or wood, or cutting 2by2's up, it's just fine.