Rainwater diverter for 40mm (greenhouse) downpipe

[kludge]

Hi,
Am just about to take delivery of a lovely new greenhouse, complete with nice aluminium gutters and downpipe. I would love to fit a downpipe rainwater diverter to this, so I can harvest the water in my water butt.
But can I find a diverter to fit a 40mm downpipe anywhere? Nope.
I'm looking for something like this:
http://www.reuk.co.uk/Rainwater-Diverter.htm.
Has anyone had any success or know of a product I could buy to do the job?

 

[RedHerring2]

If you weren't harvesting the water, where would it go?
I suspect straight on the garden, so feed all the water into the butt and direct the overflow to somewhere acceptable.