W. Baird in "The One Pound Note" (1912) advises that in the year 1820 it appears that the number of forged £1 notes presented to the Bank of England was no less than 27,993, which with 217 £2 notes and 873 of £5 and upwards, made a total value of £33,602; while the total amount of forgeries for the ten years prior to that date was altogether about £200,949.
Perhaps of equal interest is the sender of the letter.
Robert Warren was born in Scotland although as of yet I cannot say where. He is noted for being a manufacturer of Blacking with this perhaps an essential product of the day and was a liquid preservative used for keeping black leather supple or for colouring iron products such as stoves or fireplaces. I can certainly recall as a child watching elderly relatives apply Blacking to the fireplace.

To simply describe Robert Warren as a manufacturer of Blacking does him a great disservice. From his premises in the Strand, Robert Warren ran one of the most successful businesses of his day. To quote from Warren's own copy, his blacking was 'sold by most Venders of Blacking in every Town in the Kingdom, in Pots, 6d. 12d. and 18d. each'. Large fortunes were to be made in this business and the manufacturer Charles Day, of Warren's rivals Day & Martin, accumulated the enormous sum of £450,000 from the trade by the mid-1830s. The success of such companies was heavily dependent upon their advertising and historians of advertising have long acknowledged that the most visible campaign of the early nineteenth century was for Warren's blacking (or 'Jet' or 'Japan' as it was also called).
Warren ran a series of ground-breaking campaigns in favour of its product, extolling it in a nationwide series of newspaper advertisements, puffing it in handbills, saluting it in advertisements painted on the side of metropolitan buildings and praising it in letters two feet high daubed on fences at the road side in the country. Sandwich men carried Warren's placards and advertising vehicles trawled the streets hailing the quality of his 'brilliant jet'. Warren is said to have been amongst the 'best advertisement writers' of his time.
It doesn’t stop there for Warren. He was also a poet in his own right and believed that other poets and writers would have the best ability to write his product “puffs” and he appears to have been the inventor of the “strap line” or “jingle”. The success of his campaigns was considerable and in the early 1800s tokens were stuck in celebration of his activities. The following is the only illustration I can find and is borrowed from an auction website. How I wish I had seen this prior to the auction.

Robert Warren’s London (Middlesex) copper halfpenny token undated. Obverse: Warren’s bare-headed bust to the right: “R. WARREN. THE INVENTOR OF JAPAN LIQUID BLACKING”. Reverse: A bottle of Japan Liquid on which is inscribed: “R.W No.14 ST. MARTINS LANE – LONDON”, with legend: “ROBERT WARRENS. LIQUID BLACKING MANUFACTORY”.
Warren’s use of poets and writers led to a satirical review of his activities and to the book “Warreniana” which was published in February 1824 by an anonymous author. The book purported to contain ringing endorsements of the well-known manufacturer of blacking (i.e. boot polish), Robert Warren, by many of the leading literary figures (Byron, Coleridge, Scott and Wordsworth amongst them) and journals (Blackwood's, John Bull, the New Monthly) of the day. It was the work of the precocious twenty-four year old London journalist William Frederick Deacon. The book offers a series of agile and vivacious parodies of a wide range of Romantic period writing: poetry, essays, literary and political journalism, historiography, sermons, parliamentary reports and scholarship – all containing humour directed at Warren.
Warren has further links to literary history as Charles Dickens, as an impoverished boy consigned to the Blacking factory as a consequence of his Father's imprisonment for debt, worked 10 hour days in the business originally owned by Robert Warren's brother and rival, Jonathan.

Dickens at the Blacking Warehouse. Charles Dickens is here shown as a boy of between eight and twelve years of age, working in a factory.
It is clear that both establishments regularly employed poets to help them sell their wares and in an obscure diary of events for the year 1833, an acquaintance asserted that Dickens himself was one of their numbers. Whether Dickens wrote for the Warrens included them in his works. or not he certainly
Halfway through the serialisation of The Pickwick Papers in July 1836 the servant Sam Weller is introduced, preparing a customer's boots "with a polish which would have struck envy to the soul of the amiable Mr. Warren (for they used Day & Martin at the White Hart)". A few months later, in a slum sketch of London's "Seven Dials", a shabby-genteel man is described as leading a "life of seclusion in a second-floor room, buying only coffee, bread, pens and "ha'porths of ink" - "his fellow-lodgers very naturally suppose him to be an author; and rumours are current in the Dials, that he writes poems for Mr. Warren".
The idea of an impoverished man of letters writing advertising poems for Mr Warren recurs in 1840, when Dickens introduces the down-at-heel "Mr. Slum" as the itinerant poet in The Old Curiosity Shop who tries to sell Mrs. Jarley an acrostic advertising her waxwork. "The name at this moment is Warren," he concedes, "[but] the idea's a convertible one".