Dave, you tank looks like one of these to me. http://www.gasapplianceguide.co.uk/Newark%20Cylinders/Newark%20Combination%20Copper%20Cylinders.htm
There might be a T piece on the cold water inlet with a draincock leading away from the bottom of the tank if you're lucky.
If not, I would undo the plastic elbow on the hot water outlet near the top and push a length of hose pipe into the cylinder through this outlet.
When you think the hose pipe has reached the bottom of the tank, syphon the water out to a drain somewhere or your garden.