The answer depends on what you've actually agreed.
If you accepted a contract back in June to pay a given price and it was invoiced to you at that point then the VAT is correct and no refund would be due (whether you've actually paid yet or not is irrelevant).
If you accepted a quote back in June but it was to be invoiced to you in April 09 then the VAT rate change is relevant and the price should drop accordingly. Again, regardless of whether or not you've paid a deposit towards it.
If the quote given included prices of alcohol and didn't have a clause about potential rises due to increased duty then I don't believe they are entitled to change that price.
To directly answer your question, unless they've already invoiced you, they CANNOT refuse to pass the decrease in VAT on. Whether they can pass a change in the price of alcohol on to you or not (it's not directly relevant that this may or may not be due to an increase in duty, the hotel itself does not pay that, the cost of drink to them will have changed of course but it may well have done anyway between June 08 and April 09) depends on what you contractually agreed with them at the time. If they quoted a price for alcohol then no, I don't believe they are entitled to change it just because duty went up.
Of course, whether you wish to make a fuss over 2.5% of alcohol is up to you? Also note, the extra alcohol duty is obviously only on alcohol. I presume you are paying for venue hire, a meal, possibly entertainment, etc. All of that should have a reduction in VAT too but no increase as they have nothing to do with alcohol duty. There's no way even if you accept the argument that alcohol duty is relevant that the increase in that can be the same as the decrease in VAT. Either way there should be a reduction (unless as I said it was already invoiced to you).