Part of the advice from the Eden Project on composting:

5. Don’t put the wrong stuff in

Certain things should never be placed in your bin. No meat or dairy products unless you’ve opted for a digester. No diseased plants, and definitely no dog poo or cat litter, or babies’ nappies. Putting any of these in your compost will lead to unwanted pests and smells. Also avoid composting perennial weeds (such as dandelions and thistle) or weeds with seed heads. Remember that plastics, glass and metals are not suitable for composting and should be recycled separately.