Have you tried posting on Netmums? Worth a shot if you haven't as they do tend to have more local stuff/people.
Have you called Family Information service? They can post or email you a list of childminders. The number is 01482 318318.
Also hull childminding network (tel 01482 588808) may have details of childminders in the process of registering which FIS wouldn't know about, they also have vacancy co-ordinators in each area of Hull.