should stainless steel cooker splashback be earthed?

There's no legal requirement to bond it (unless it is part of the appliance). Given the large area, I would consider bonding it, if there is any conceivable way it could get an imperfect earth by some means. This could be if it touches a metal cased earthed appliance (including the cooker), or metal pipework. If it only conceivably touches insulated items, it may be safer to leave it unbonded.

Christian.

There's no requirement for earth bonding in a kitchen. Some people do it, and others don't. Personally I do within reach of the sink. It's not universally held to be a good idea though -- there are some failure modes where it makes things more lethal.

I suppose Agas are another thing like this - ours doesn't do hot water any more (although it did, and still has the remains of some 1" pipe hanging around in the voids somewhere) and is connected to the gas supply. So it's probably "vaguely earthed".