Mould is a HUGE problem in many of the rental units in London. I came across this comprehensive article about this issue here: The Tenant’s Guide to Mould. At the end of this article, there's a list of more helpful links on the same topic. Knowledge is power!
If you, or a tenant you know is having a mould problem, here is a fantastic article about how to ask your landlord to look after this problem. How to Ask Your Landlord to Fix a Mould Problem
Finally, for everything you need to know about the law which applies to rental properties, you can check out the CLEO website. Here's the link about Maintenance and Repairs.
I discovered the first link on http://www.shscorp.ca/ - The Social Housing Services Corporation. Check out the Resources link for more helpful informotion.