No way was I wasting 20kg worth of cat food though. I wrestled the bag outside and blasted it with the hose to at least get the ants off the outer surface before I opened the bag, and then decanted the food into my big plastic canisters. The ants seemed to be concentrated at the very bottom of the bag; only one small canister ended up with the odd ant in it and I was able to fit it into the freezer which should fix that problem.
But ever since I keep finding ants on me. It was a couple of hours ago now but I'm still picking the occasional ant off my arms. Rotten little buggers.