As far as I'm aware theres nothing to say you can't fill up at the low duty rate and send a declaration letter and cheque direct at the end of the year when you know the percentage you have used for propulsion, after all you can fill out one declaration for the whole year if you use the same supplier each time and the supplier is only required to submit funds/declarations annually, the 60% is only a guideline for guess work, if you genuinely use more than 40% for domestic you can declare any percentage you want, I know live-aboards that declare 0% propulsion as they never move anywhere other than the nearest pumpout facility (which is still domestic use).
Keep a log book of miles travelled and fuel used for heat/light/water and your covered.
I've had red delivered to the house in large quantities and as soon as you say marine to the driver they don't care.
It may be worth a call to your local oil supplier to see how they stand on IBC tanks, I think up to 2500 litres doesn't have to be bunded, my 1000 litre heating oil tank isn't bunded and thats never had an issue with deliveries, a bit of common sense on positioning near water courses and drains wouldn't go amiss though.