Hi Glen
I don’t have a DIY trailer system, but I believe you would be able to build one for in or around £600 excluding the price of the trailer. This would give you a 250lt water tank. It would be very easy to put together. I could give you a list of what you need and how to install it all, if required.
If however you had 2.3k to spend, Ionics do a really nice trailer system which also comes with a 18ft residential pole.
Andrew