From their site
This month's offer is a free E.ON ShowerSave*. It's available to customers signed up to 10:10 and could save you up to £40** per year on your utility bills.
Helps to reduce the amount of water you use
Still allows you to have a decent shower
Is easy to fit and only takes a few minutes, just install it between your shower and hose
Comes with a 15 year guarantee on flow rate control element
ShowerSave .. it was there 2-3 months back and then disappeared It seems is back on e-On.
I think you have to be existing customer and be a part of their 10:10 campaign to order one.