This quote from the standard nearly says it all: "the seriousness with which the fault should be regarded should be in exact proportion to the degree and its effect on the terrier's ability to work"
British Breed Standard (1986)
General appearance Essentially a working Terrier
Characteristics Capable of following a horse, combining activity with gameness
Temperament Active and game as previously stated
Head and skull Head like that of an otter but moderately broad in skull, with short strong muzzle. Black nose preferable, but liver- or flesh-coloured one not a serious fault.
Eyes Dark, with a keen expression.
Ears Small, V-shaped; of moderate thickness, and dropping forward close to the cheek
Mouth Scissor bite, i e upper teeth closely overlapping lower teeth and set square to the jaws. Level bite acceptable. Undershot or overshot a major fault and highly undesirable.
Neck Of moderate length
Forequarters Forelegs straight, not too heavy in bone
Body deep, narrow, fairly long. Ribs carried well back, but not oversprung, as a terrier should be capable of being spanned by both hands behind the shoulder. Loins strong.
Feet Small with thick pads.
Tail Moderately short; fairly thick at the base, then tapering. Set high, carried gaily, but not curled over back.
Gait/Movement Has the soundness to follow a horse.
Coat Harsh and dense; with close undercoat. Skin must be thick.
Colour Red, wheaten, grizzle and tan or blue and tan.
Size, weight Dogs: 5.9 - 7.1 kgs (13 - 15.5 lbs); bitches 5.1 - 6.4 kgs (11.5 - 14 lbs).
Faults Any departure from the foregoing points should be considered a fault and the seriousness with which the fault should be regarded should be in exact proportion to the degree and its effect on the terrier's ability to work.
Note Male animals should should have two apparently normal testicles fully descended into the scrotum.