Per Ward LJ in Stannard (t/a Wyvern Tyres) v Gore  EWCA Civ 1248 at paras 26-31:
26. The custom of the realm is that a person is liable for the damage caused by the escape of his fire – the ignis suus rule. By general custom of the realm the remedy for damage caused by the spread of fire was an action on the case pur negligent garder son few. The declaration was in the form "whereas by the custom of England a man is bound to keep his fire ignem suum safe and secure lest by default of custody of it loss should fall upon his neighbours in any way, the defendant tam negligenter ac improvide apud custodivit, quod pro defectu debitae custodiae the fire spread and did damage …".
27. The essential features are that (1) fire spreading from a person’s land is his fire; (2) that person is under a duty to contain the fire: (3) if it escapes, he is liable for the damage that results; (4) "negligenter" was a pleader’s adverb: it did not mean negligently in the sense we now use the word. To quote Winfield again (Winfield: The Myth of Absolute Liability), (1926) 42 L.Q.R. 37, 49:
"Exactly what it did mean must remain a matter of conjecture but it excluded liability when the fire spread or occurred (i) by the act of a stranger – a man was not liable for that, though he was for the act of his servant, his wife, his guest, or one entering his house with his leave or knowledge; (ii) by misadventure which, to anticipate a modern term, seems to be equivalent to inevitable accident, or something which by no care reasonable in the circumstances could have been avoided."
28. The earliest case on the subject is Beaulieu v Finglam (1401) B. & M. 557 where, as reported by Fifoot History and Sources of the Common Law, 1949, Markham J. said:
"A man is held to answer for the act of his servant or of his guest in such a case; for if my servant or my guest puts a candle by a wall and the candle falls into the straw and burns all my house and the house of my neighbour also, in this case I shall answer to my neighbour for his damage.
I shall answer to my neighbour for each person who enters my house by my leave or my knowledge, or is my guest through me or through my servant, if he does any act, as with a candle or aught else, whereby my neighbour’s house is burnt. But if a man from outside my house and against my will starts a fire in the thatch of my house or elsewhere, whereby my house is burned and my neighbours' houses are burned as well, for this I shall not be held bound to them; for this cannot be said to be done by wrong on my part, but is against my will."
29. Moving forward nearly three centuries to Turberville v Stamp (1702) 12 Mod. 152, a fire started by the defendant in his field was held to be as much ignis suus as one lit in the house. Sir John Holt C.J. said:
"Every man must so use his own as not to injure another. The law is general; the fire which a man makes in the fields is as much his fire as his fire in the house; if it is made on the ground, with his materials, and by his order; and he must at his peril take care that he does not, through his neglect, injure his neighbour: if he kindle it at a proper time and place, and the violence of the wind carry it into his neighbour’s ground, and prejudice him, this is fit to be given in evidence. But now here it is found to have been by his negligence; and it is the same as if it had been his house."
These were times when fires were a scourge. The Great Fire of London in 1666 had had a devastating impact on crowded urban living. Something had to be done. In 1707 the Fires Prevention (Metropolis) Act (6 and 31) was passed "for the better preventing the mischiefs that may happen by fire". Provision was made for improved safety, for the control of fires and for penal sanctions on servants carelessly setting fire to houses. The Act was repealed but re-enacted, eventually as the Fires Prevention (Metropolis) Act 1774, section 86 providing that:
"No action, suit, or process whatsoever shall be had, maintained, or prosecuted against any person in whose house, chamber, stable, barn or other building, or on whose estate any fire shall accidentally begin, nor shall any recompense be made by such person for any damage suffered thereby, any law, usage or custom to the contrary notwithstanding."
It may be that, as Sir William Holdsworth in his History of England vol. XI p. 607 speculates, by this time lawyers were beginning to think it anomalous that a man should be liable for fire damage not caused by his negligence. It may have been that Parliament was simply resolving any doubt there was as to whether fire spreading from a person’s house was ignis suus. Another explanation is implicit in Lord Tenterden’s judgment in Becquet v MacCarthy  2 B. & A.D. 951, 958:
"… By the law of this country before it was altered by the statute of 6 an. c.31, s.6, if a fire began on a man’s own premises, by which those of his neighbour were injured, the latter, in an action brought for such an injury would not be bound in the first instance to show how the fire began but the presumption would be (unless it were shown to have originated from some external cause) that it arose from the neglect of some person in the house."
Thus one can surmise that the purpose of this statute was to remove the presumption of negligence on the defendant’s part and cast the burden of proving negligence on the plaintiff.
31. Curiously there seems to be little contemporaneous authority on what was meant by "accidentally". Lord Lyndhurst flirted with the idea that accidentally was used as the antithesis of wilfully: see his obiter remarks in Canterbury v A.-G. (1843) 1 Phil. 306. The more informed view is expressed by Lord Denman C.J. in Filliter v Phippard (1847) 11 QB 347, 357:
"It is true that in strictness, the word accidental may be employed in contradistinction to wilful, and so the same fire might both begin accidentally and be the result of negligence. But it may equally mean that a fire produced by mere chance or incapable of being traced to any cause, and so would stand opposed to negligence of either servant or masters. And, when we find it used in statutes which do not speak of wilful fires but make important provision with respect to such as are accidental, and consider how great a change in the law would be effected, and how great encouragement would be given to that carelessness of which masters may be guilty as well as servants, we must say that we think the plaintiff’s construction [accidental as opposed to negligent] much the most reasonable of the two."