You are going to need to do a lot of hours to make it worthg storing your own diesel. Even the most expensive marina is unlikely to charge more than 35p/litre, and with a bit of shopping around you should be able to get it for about 25p/litre -- even in the Solent.
The best price that I know of is in Cowes, just past the floating bridge on the West side of the river.
You will also find that it is quite easy to strike a deal with a marina if you fill up there regularly, even if you don't set up an account.
Always haggle -- most places do charge an inflated price for casual visitors!