Hi Fauve,
From personal experience, I cancelled an MBNA credit card in late 2007, and they gave me a letter stating I had paid the government duty (same as you got from BOI) for 07/08.
As far as I know, if I had, after closing the MBNA card, opened another card in the same chargeable year (ie before April 08) that letter would have meant I wouldn't have been liable for the second credit card fee, until after the first of May 2008, in which case the fee would be for 08/09, and shouldn't be charged until 2009.
This is not the case if you have 2 credit cards on the go at once , as you would be liable to pay twice then.
I think from what happened to me, it seems that Halifax may have made an error with your account, unless for some reason they were charging you in advance, not in arrears for 2008/09 (incorrectly).
You need to contact them to ask (and again, get it in writing:I advise this after my own experience of a long paper trail from MBNA, about exactly the same thing you are asking about, which is still waiting to be resolved. At least everything is in writing though, so there is proof that all was paid up to date)
Nicola