Hertford Spar refuses to give me declined slips when their systems are down

I went to the Hertford Junction Kwikspar, which I have been using for several years, and their card machines declined my card due to the service being down. Knowing that this frequently causes the bank to lock the money in a holding account until you have furnished them with the appropriate form and the declined slips for every declined transaction, I asked for my declined slips. The teller said that she had never heard of it and couldn't give me my declined slips. The manager was called and he also said he had never heard of such a thing and they couldn't give me my slips until I paid. Only when nearly 15 minutes later I insisted that they cancel the transaction, and give me my declined slips, did they give me them. I fully understand that the locking of funds in a holding account is a problem with the banks and not the shop, but I feel it's very unreasonable for the shop to refuse to release the declined slips until I have paid, even though they have no means of allowing me to pay as their systems are down. I feel that my initial request to cancel the whole transaction, keep the groceries and furnish me with the declined slips so that I can release my funds from the bank should have been comp**** with without resistance.