Can I spur off the cooker spur to feed a gas hob?

If it's on a 40 amp mcb with a 10mm cable, then no. A 10mm cable wouldn't go in a fused spur. If it's on a 32 amp with 6mm cable it would be fine. 6mm from outlet to spur. Then 2.5mm out of the load to a socket. If it is 10mm. Just spur off one of the kitchen sockets. Hope that helps.

A gas hob igniter is negligible load. It is normally plugged in to a socket. The circuit you describe is standard for a domestic cooker having a 6mm2 T&E on a 32A mcb but you say it goes in to a junction box? then to a double socket which is also spurred to a fused spur? It seems that the circuit has already been altered. It would be more usual for the cable to go from the Consumer Unit to a cooker control switch, then to an outlet plate where the oven and/or hob would be attached. If the fused spur is in the right position you could come off that but it depends on what the spur is being used for and what size fuse is in it. Its difficult to answer questions like this because we can't actually see the configuration of the wiring and how it is connected. Technically it shouldn't be a problem and would be safe. Its a pity we cant see photographs.

Going from one size of cable to another needs to fused down for protection if the cable

The cable size must match the fuse size, ie the bigger the cable the bigger the fuse. Bigger is best but most expensive. You could feed your igniter with a 1mm cable but must fuse down accordingly. I would feed my spur with 6mm and output from spur with 1.5 with a 3 amp fuse in the spur. Ideally wire your spur from a socket.