we're just fitting a new kitchen, and intend to have a stainless
steel splashback behind our dual fuel (gas hob/electric oven)
cooker. There's chimney style extractor hood above.

Should it be earth bonded? (by law and/or common sense)

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.