The "threat" in this case looks like one of those instances where the dictionary meaning of a word and its legal meaning lean in different directions. Of the letter which constituted the alleged threat, the judge says, at paras 96 onwards:
"i) The letter did not mention proceedings or explicitly threaten them;
ii) The letter asserted that "up to this point" the writer could see no difference in the technical solution; but contained a request for the reason why the recipient thought that it need not "take into consideration" the patent;
iii) The writer of the letter was not an English lawyer to whom the conduct of proceedings about patent infringement in England would usually be entrusted.
iv) On the other hand, the letter was not addressed to the manufacturer of Zeno, nor even to Boots head office. It was sent to the stores that were actually stocking the product.
The packaging in which the Zeno product was sold displayed the name and address of the manufacturer in Houston Texas and its UK distributor.
The writer of the letter knew who had manufactured the Zeno device; who had imported it and who was offering it for sale. So the purpose of the letter cannot have been to discover who was infringing the patent (if indeed it was being infringed).
Was the "sole purpose" of the letter to discover whether the patent had been infringed? If the writer of the letter were genuinely concerned to ask about some technical characteristic of the Zeno device that would point out a difference between that device and the device described in the patent the obvious person to write to would be the manufacturer. Why should the retailer know the technical details of what it sold? And even if the retailer did know, surely the relevant person would be someone in head office responsible for Boots' corporate decision to stock it rather than any individual store manager. Why, then, were the letters sent to the retailers? The only answer must be to persuade them to stop selling the Zeno device. It partially succeeded in that objective. In my judgment that means that it cannot be said that "the sole purpose" of the letter was to discover whether the patent had been infringed.
It is true that the letter made assertions about the patent; but it also made assertions about the Zeno device.
Mr Burkill objected that no evidence had been called from Boots to show how they interpreted the letter. I do not think that matters. In Mannai Investment Co Ltd v Eagle Star Life Assurance Co Ltd  A.C. 749 the question was whether a notice given by a tenant to a landlord was valid to terminate a lease. Lord Steyn said:
"The question is not how the landlord understood the notices. The construction of the notices must be approached objectively. The issue is how a reasonable recipient would have understood the notices; and in considering this question the notices must be construed taking into account the relevant objective contextual scene."
In my judgment the same applies here. It is not a question of how Boots understood the letter; but how a reasonable person in the position of Boots would have understood it. Read in context, through the eyes of a retailer, the letter amounted, in my judgment, to a veiled threat of infringement proceedings.".The judge appears to have got it right here, but the Patents Act 1977, s. 70, which deals with threats, has never been particularly easy to apply. PatLit wonders if it would be helpful if the Intellectual Property Office posted a set of general guidelines, based on judicial rulings, that litigators can refer to, perhaps also with actual examples of letters to suspected or alleged infringers that have been found to constitute threats and those which have not.