Well today we went to see our 2nd/3rd rental feild and it's a go! 4 acres for £200 a year plus a leg of lamb. I'd say that's okay? It's got water and stock proof fence so after my holiday I'll be getting my first sheep! I almost wish the holiday was over already so I can get them now!
4 ac could keep about 14 sheep in summer in a good year and about 6 in winter. So; if we assume that 'summer' and 'winter' grazing make up half the year each, it is worth 14 x 26 (weeks) @ 20p/head/week = £72.80 + 6 x 26 (weeks) @20p/head/week = 31.20 is £104 for the whole year.
However, the owner of the land will struggle to find a commercial sheep farmer interested in grazing it (unless he/she lives next to one) - so I'd say pay £100 if you want, but all the grazings I have of that size I graze for no rent and give the owner a lamb (for the freezer) each year.
At 200 quid, assuming you lamb at 180% and have winter grazings elsewhere for your 14 sheep rent free, each lamb has already cost you £8 to produce, before meds, feed etc.
However, it can be hard to get grazing in the first instance, so it might be worth it to get yourself known and get offers of further grazings.