Some marinas and boatyards don't sell that much fuel in comparison to fuel boats which might effect the quality of fuel you get. Fuel boats have a very high turnover of fuel and in the case of Jules I know they also add a fuel additive so you don't have to. Their diesel price is competative and you can sign up for text alerts so you will know when they are passing. My experience has been that LPG, coal etc. may be more expensive from a fuel boat, but you do get the convenience of not having to cart the stuff down the towpath.