Hi Ian,
My language school has their own policies for students. It's about €30 p/m I think. I'm doing bits of work as well so I've signed up for a policy with Hanse Merkur. (http://www.hansemerkur.de/produkte/reiseversicherung/gaeste) It's a 12 month policy specificialy for foreigners who are planning to be in Germany for less than 5 years. The basic policy is €69 euros p/m, which is by far the cheapest private policy I've come across here, apart from the language school.
The
Health Insurance here is an absolute nightmare so prepare yourself for some serious stress. Just a bit of a warning!
p.s. If you're British the NHS covers you under the EHIC card for 12 months as a student here, if your course is not recognised by a British Institution. If it is recognised then you're covered anyway until your course ends. Zou should call the NHS though and let them know you're here if you plan on doing that. Sorry I don't have the number.
Hope this helps.