Dave
Billhooks are generally sharpened with a "Scythe or canoe Stone" They are usually available from where you purchased your billhook, or farm suppliers
Look here for how to do it... http://handbooks.btcv.org.uk/handbooks/content/section/1725
And here for a supplier
http://www.justoffbase-tools.co.uk/tools.asp?parent=2I6202P1814M0
The BCUK Dead Sea Scrolls!