john west wrote:

Could someone enlighten a non technical person about what fittings i can
get, that will enable a plastic garden water tank to be drained by
connecting it (securely and non-leaking) to a 20 metre length of
standard (1/2") garden hose please.

Either a 3/4" or 1" tank connector, together with a hozelock type connector

http://www.amazon.co.uk/dp/B00KKNLGAA
http://www.amazon.co.uk/dp/B0056GS72Q

If you want a tap to turn off the hose, then this might suit better

http://www.amazon.co.uk/dp/B00GMCYPA6

Be aware you'll need access to the inside of the tank to screw on the
back-nut.