I buy the huge cube Rockies ones. They do several different ones depending on what sort of supplement you want; they are absolutely rock hard so the horses can't "eat" them; they last for AGES. Mine love theirs.
I also use the Rockies from Countrywide. The licks don't get washed away easily in the rain and they last for ages. I bought my last lot over six weeks ago and still loads left on them.
Paddock lick from dengie massive will last you 4 weeks at least for one horse dengie alfa-alfa based and not sticky like the likit one etc you can get the tubs for them as well and then you just get one about £10.00 each. I use them over the winter when the fields are a bit sparser.
After much phoning around I got the Dengie Paddock Lick - horse had a few chomps at it and is now licking it when bored (which was the idea!)
Thanks folks