pico:
The attiny85 can go up to 16MHz without an external xtal. The maximum 20MHz requires an external xtal, though.Its just a matter of selecting the clocking fuse values appropriately. I run my attiny85 chips routinely at 16MHz, sometimes (more often than not, actually) even at 3.3V (shock horror!
)
Thanks for setting me straight on that. I just fired up the IDE and sure enough, there's the internal 16MHz option just above the one I typically choose. All this time I'd thought that was an external flavour...I'm going to use male pattern blindness as my excuse.
Thanks again 
Geoff