Too much water is always a prime suspect when the leaves of rosemary turn brown, but other things, like an excess of soluble salts in the soil or insect infestations (scale/mites usually) can be the cause, too.

The plant hates wet feet, appreciates a very fast draining soil, does best in full sun, and seems to appreciate fertilizers with an NPK RATIO (different than NPK %s) of 3:1:2.

Al

Many thanks for your help which covered a lot.