Codprawn is right. There is only ULS petrol in the country (no unleaded at all) and thus you must fill in the ULS section of the HO50 form.
This confuses many as what you buy at the pumps says unleaded.
Dont forget to claim the VAT on the petrol and then the duty bring the litre price to under 20p.
There is a thing called adval in Customs and an inspector with little knowledge of duty rebates may challenge the amount of VAT you claim. If you think about it you are claiming the amount of VAT on the whole price (ie. Pertol cost plus duty).
The good news is that in the case of petrol this is quite acceptable ie. you claim the full VAT on the full purchase price.
Other rules can come into play such as to claim Duty it is expected (though I am not sure whether it is in statute) that you are VAT registered. From that to zero rate your passenger supply you are expected to have a vessel licence for a minimum of 12 passengers.
And for those who use diesel and may not be very interested on these points then you may be interested to know that there is 4p duty on red that you can claim.