Anyone used them?
http://www.plasticwatertanks.co.uk/1.html
Neil at EMPF provides an excellent service and I have used him many times for custom fabrications.
In the back my van I still use a hose reel drip tray he made for me about 10 years ago. I also have a custom made pole holder which he made, which bolts to the top of my fitted system.
These tanks really last. I had a custom tank made about 9 years ago for an estate car and it is now on its 4th owner and about its 6th vehicle.
Send him a drawing of what you want and he provides a quote within days and usually fabricates within 3 weeks.
I have always found him good value for what is a very bespoke service. He is also happy to arrange delivery.