My recollection is in accord with Geoff the Duck. Having lived many years in Bradford and before that in North Yorkshire I understand a ginnel to be a snicket with a roof. So a ginnel is, for example, the passageway through a row of back-to-back houses. A snicket is, as Geoff says, a longer path or lane but open to the elements. In Leicester, where I now live, both are called jittys or jittyways.