In reply to JdotP:
I'd be cautious in your shoes to be honest. A C.1900 house will have lime mortar and is very flexible. For a crack to go the full height of the gable means something has moved significantly. This could be old settlement, or a new problem. The wall is unlikely to have a concrete foundation, but rather be corbelled (I.e. stepped) brickwork built off the sub-soil widening out to support the structure. Normally any movement is absorbed by the relatively flexible form of construction, and would take place in the first few years. (how old is the crack? Can the vendor prove it's old?)
Things I'd consider in your shoes:
Is there a cellar? (Has a cellar been filled in??
Any cracking on the front or back elevations (I.e. is one of the corners on the move?)
Broken drains (Drain survey?)
Nearby trees causing shrinkage (Is the house built on clay?)
To echo comments above, a structural survey will be caveated; the surveyor will point to the crack and highlight it; you'll have no comebacks on them.
I'd get a good local builder to have a look.