Thank you Barry and Fred for your helpful replies.
Fred I thought this would be the correct way of doing things but the reason I asked the question is because I am sure that when I opened up the drain in the garage boot not only did it drain the pipes but it also drained the tank. Now I am not entirely sure of this so at some stage when I get time I will fill the tank again and see if it drains with the tap opened in the garage.
I will let you know what happens.
Once again many thanks for your help.
Dougie