repairing crumbling cills

[dannyr]

All the external cills on victorian house are crumbling badly - tops, fronts and also the bottoms are uneven and partially missing. What's the best way to repair these. I've scraped them back to decent clean stone. Do i need to build a wooden casing around them and pour in new material? What should I use that will bond to the old cill? Do I need to apply something to the old, cleaned, cill before adding the new material?
Done a lot of googling and searching this site but I cant find instructions for this job.

thanks
dan