I picked up a tip from the Polymer Clay Forum yesterday. Lidl have small blade sharpeners @ £2.99 each, just the right size for sharpening tissue blades. Warning get a pack of plasters at the same time. I used it on my bluntest blade, shortly afterwards I was using the blade, had it upside down and cut myself. Am always using them upside down, normally only damage done is to the cane.
Need to be quick, new offers start today and they may not be around for much longer.