This is still in development, but have a look at;
http://dieselcalculator.110mb.com
I've made a form that calculates the total due depending on what split is declared. This way, I figured, your boatyard can't claim it's too much work for them to calculate.
I've based it on the maths on the HMRC site, but if anyone disagrees with my maths, please do let me know.
I'll sort out the formatting, prettify it, and whatnot, but at the moment - any thoughts, feedback or comments would be gratefully received...