Polytunnel Guttering - a big step towards solving the water shortage?

Debbie and I have just spent a lovely couple of days working on a little allotment project together, as a spin off we've made some real progress towards alleviating potential water shortages in early spring and late autumn. We will still need supplemental water in late spring, summer and early autumn, especially if we have another drought!
Here's the background to the project:
Our council is strongly encouraging us to save as much water as possible and for me this means finding a way to collect water from the polytunnel and greenhouse. This video shows what we've achieved so far.
Before anyone else rushes out to do the same though, it's important to note that it's also a very economically dubious project, because it will take over 12 years to pay back the investment, and I don't even benefit from the saving - the council does! In fact 12 years is probably longer than the solution will last, but I try not to think about that!
The cost of the extra water usage this year - due to the drought - was about £5 and I have effectively begged the committee to put the rent up to cover this or to accept a £50 donation to cover the total costs of water several times over. In my desperation I even offered - only half jokingly - to pay the sites entire water bill (160 plots) and recover my investment through a voluntary - honesty box - type system. This would relieve the council of the pressure to reduce their costs by a tiny amount, while increasing the risk to our harvests by a huge amount.
However for some good reasons they are not interested in these solution, so we have no option but to spend money to save water. The cost isn't insignificant but set in the context of the value of the crops we harvest it's trivial, so I'm 'happy' to make the investment.
For content the cost of the tap water that we used this year to water the allotments was 0.5% of the value of the crops harvested, so it should be a no-brainer to put up the rent.
Finally I know lots of people don't have any tap water at all on their sites, so I'm very grateful that we do, otherwise we would be in real trouble with the types of water loving veg we grow all year round.
If you are new to my allotment videos you might find a bit of context useful. We have three allotments in my family, mine (Steve), my wife's (Debbie) and my middle daughter's (Jennie). We also have a small kitchen garden at home. They are all managed in an integrated fashion, so don't expect to see the usual mix of veg on each plot.
On Jennie's plot, for example, we focus on potatoes, squash, alliums, and brassicas. This video provides an overview • The big picture of our... . I do an update of the allotments, roughly one a week, you can find the tours here • Tours .
Our approach to allotment life is to: grow as much as we possibly can, to be self-sufficient in veg all year round and in fruit in season, to give away our huge surplus to friends and family, and to have as much fun as possible.
My wife and I spend about 4 hours a day, 4 days a week on the plots (on average) and we keep nudging that down as we eliminate non-productive work: like grass cutting, weeding and watering as much as practical. We are both newbie gardeners, only starting the allotments in 2016.
I'm a bit obsessive about the nutrient density of the veg that we grow and making the plots easy to work because it's through this allotment lifestyle and food that I've overcome a debilitating auto-immune disease.
I'm always aware though that it might not last so I make sure that I don't work too hard, eat as much organic fruit and veg I can and design the plots so that I can still work them if I flare up again.