This is taken from the protection of livestock Act 1953....The important bit relating to sheep i have highlited in red. A dog not on a short lead or at heel, whether it is bounding about or just sniffing around is 'out of control' and this bit is for sheep only.....the big problem comes further down where it mentions the dog being with someone not its owner.
Penalty where dog worries livestock on agricultural land.E+W(1)Subject to the provisions of this section, if a dog worries livestock on any agricultural land, the owner of the dog, and, if it is in the charge of a person other than its owner, that person also, shall be guilty of an offence under this Act.
(2)For the purposes of this Act worrying livestock means—
(a)attacking livestock, or
(b)chasing livestock in such a way as may reasonably be expected to cause injury or suffering to the livestock or, in the case of females, abortion, or loss of or diminution in their produce.
[F1or
(c)being at large (that is to say not on a lead or otherwise under close control) in a field or enclosure in which there are sheep]
[F2(2A)Subsection (2)(c) of this section shall not apply in relation to—
(a)a dog owned by, or in the charge of, the occupier of the field or enclosure or the owner of the sheep or a person authorised by either of those persons; or
(b)a police dog, a guide dog, trained sheep dog, a working gun dog or a pack of hounds.]
(3)A person shall not be guilty of an offence under this Act by reason of anything done by a dog, if at the material time the livestock are trespassing on the land in question and the dog is owned by, or in the charge of, the occupier of that land or a person authorised by him, except in a case where the said person causes the dog to attack the livestock.
(4)The owner of a dog shall not be convicted of an offence under this Act in respect of the worrying of livestock by the dog if he proves that at the time when the dog worried the livestock it was in the charge of some other person, whom he reasonably believed to be a fit and proper person to be in charge of the dog.
(5)Where the Minister is satisfied that it is inexpedient that subsection (1) of this section should apply to land in any particular area, being an area appearing to him to consist wholly or mainly of mountain, hill, moor, heath or down land, he may by order direct that that subsection shall not apply to land in that area.
(6)A person guilty of an offence under this Act shall be liable on summary conviction—
(a)except in a case falling within the succeeding paragraph, to a fine not exceeding ten pounds;
(b)in a case where the person in question has previously been convicted of an offence under this Act in respect of the same dog, to a fine not exceeding fifty pounds.