Nothing to do with Duty, Harvey's may pay slightly more duty on their products than Long Man because they are a larger brewery (but still pretty small in the grand scheme of things) but not enough to account for a 50% price hike. Last time I looked at prices for various Sussex breweries Harvey's...
That is what the pub charges not what the Harvey's brewery charges for that pint. A cask of Harvey's Best is nearly 50% more expensive than Longman Best (and many other local equivalents). That is why many landlords in Brighton have dropped Harvey's.