What would be the best way to approach a CS university department for interns?

I have done all my studies overseas so I still do not know how intership works in the UK.

1. 
   

   What would be the best way to approach CS university department to get interns into my organisation?

   
   


2. 
   

   Should I just email a head of CS dep?

   
   


3. 
   

   What is the interns-companies market like in terms of supply and demand?

   
   


4. 
   

   How do I make sure I attract the the best CS students?

   
   


5. 
   

   Are companies supposed to pay salary to interns or is it always free?

   
   

My questions are about UK if there is a difference to the rest of English-speaking world.