The listing below convinces me that it is pointless distinguishing between Thomson and Thompson (not a Tintin joke). Godwin may have sometimes used the two spellings to distinguish two people who appeared close to each other in the diary (e.g. possibly 18.9.1815), but he clearly also used both spellings randomly for the same person, see for example the Thomson/Thompson linked to Curran in 1817 or the two spellings of Marmaduke Thompson in 1804 and 1806. As I am working through the diary I naturally have less ideas about the later entries but they are set out below waiting for answers
In Godwin's 1796 list for 1784 as W Thomson, in 1794 version as Thomson = William Thomson DNB 1746-1817 worked with Godwin on the Political Herald
15.6.1792 Thomson at Free Mason's Tavern = quite likely William T 1746-1817
9.3.1794 Thomson at Tooke's / 10.3.1794 meet T Thomson (at or after Waghorn's with Mackintosh). In Godwin's 1796 list as T Thomson = Thomas Thompson 1767-1818 (History of Parliament) said to be illegitimate son of Levi the Jew.. MP for Evesham, Friends of the People 1792, proposed member SCI 24.1.1794, Friends of Freedom 1795 and (QV*) in Society for Constitutional Information dataset
1.11.1794 Thomson at Hardy's trial = likely Thomas T as on 10.3.1794 as next to Wharton another MP, but could be William T of 1796 list
27.12.1794 Thompson at C Moore's (with Losh, adv Gurney) = likely Thomas T as on 10.3.1794 but perhaps William T of 1796 list
22.3.1795 Smith & Thomson breakfast = same as 27.12.1794
5.5.1796 sup at Twiss's w. Opie, adv. Thomson's = perhaps Henry Thomson DNB 1773-1843 pupil & friend of Opie
12.2.1797 Thomson at Tooke's = likely Thomas T as on 10.3.1794
19.3.1797 Thomson at (Thomas) Knight's = not sure
3.9.1797 call with Montagu on Dr Thompson = perhaps, as this was at the time of the fatal illness of Godwin's wife, a medical doctor and if so likely Gilbert Thompson DNB 1728-1803 a Quaker practising at Salters Hall in the City of London
6.3.1798 Thomson at Bull's, Bath / 9.3.1798 at Bull's, then at miss Lee's. With Col Barry each time. This may also have been William T of 1796 list
14.3.1798 Thompson on Bath coach = no way of knowing
29.7.1798 Thompson at Tooke's = likely Thomas T as on 10.3.1794
20.12.1799 Thomson at Opie's = probably Henry T as on 5.5.1796
7.8.1800 dine with Thomson at Camp (in Ireland) = a militia officer not yet identified. See Bodleian Abinger c6 f36-7, Godwin's letter to James Marshall recounting his irish adventures
23.3.1802 Thomson at Davy's = not sure
7.3.1804 Dr Thomson at Joseph Johnson's = quite likely William T of 1796 list who had an honorary doctorate of laws
11.10.1804 M Thompson at Lamb's / 30.3.1806 Marmaduke Thomson at Lamb's = Marmaduke Thompson 1776-1851 Alum Cantab & Letters of Charles & Mary Lamb. At Christs Hospital school with Lamb
16.5.1805 Thomson at Wake's = likely to have been a Bath connection (see Wake, William Robert) so possibly the Thomson of 6.3.1798 and so perhaps William T of 1796 list
18.12.1808 mrs Thomson at Hume's / 31.3.1809 again / 26.7.1809 again / 25.12.1809 Thomsons at Hume's / 26.1.1810 mes Hume & Thomson call / 27.5.1810 mrs Thomson at Hume's / 22.6.1810 mrs Hume & mrs Thomson call / 5.8.1810 mrs Thomson at Hume's / 25.12.1810 again / 3.3.1811 again / 31.3.1811 again / 23.6.1811 again / 25.8.1811 again / 25.12.1811 Thomsons at Hume's / 22.3.1812 mrs Thomson at Hume's / 5.7.1812 again / 9.8.1812 again / 14.3.1813 again / 10.4.1814 again = probably Anna the 2nd wife of William T of 1796 list. In Holden's 1811 Joseph Hume Esq and Dr William Thomson were both listed as living at Kensington Gravel Pits. They were close neighbours and the Thomsons may have lived in Montpellier House with the Humes. Mrs Thomson may have been a sister of Joseph Hume or of his wife. She was almost always present (or adv) when Godwin dined at Hume's (up to 1814) and twice called on the Godwins with Mrs Hume. William Thomson may have been ill or reclusive at this time. The Godwins had three Christmas dinners running at Hume's and two of them were the only times Godwin noted Thomsons rather than Mrs Thomson. Thomson's 2nd wife was said (Gents Mag 1817/1 p647) to have been a novelist, so was probably the authoress of Excessive Sensibility 1787, Fatal Follies1788 The Labyrinths of Life 1791 (these three publ by Geo Robinson who paid Anna Thomson £40 for the last), Robert & Adela 1795, Emily Dundorne 1799, The Pride of Ancestry 1804, Laurette 1807; see The English Novel 1770-1829 Raven et al.. William Thomson's will London CC 1.4.1817 made his wife Anna his executrix and she proved the will with a neat and flourishful signature. Though I think this a good identification, I'm puzzled why mrs Thomson no longer appeared at Hume's dinners after 1814, and I can find very little genealogical evidence for the fate of Thomson's wives or children. For the Humes see Corney, Miss
10.6.1809 Thompson calls / 13.6.1809 again / 15.6.1809 again / 16.6.1809 call on Thompson
(unlikely but worth considering for some of these entries Samuel Thompson DNB 1766-1837)
21.12.1810 Thomson at Wardmote St Bride's. No-one of that name among the Farringdon Without candidates for Common Council in 1810 but there were at least 628 voters present. At the ward meeting reported in Cobbet's Weekly Political Register 26.5.1810 Samuel Thompson thanked the chair and in the Wardmote reported in the Statesman on 22.12.1812 Mr Thompson made a pointed political speech. There were several possible identities for him in Holdens directory 1811 including a callenderer of Addle-st and an oilman of Tabernacle-sq
6.10.1814 Thomson of Newburgh sups / 12.4.1816 Thomson at Writers' Library, Edinburgh. George Thomson corn merchant of Newburgh died 1853
15.9.1815 seek Thomson / 18.9.1815 call on Thompson, WM / 19.10.1815 seek Thomson / 20.10.1815 call on Thomson
8.5.1817 Thomson at Curran's / 17.5.1817 again / 7.7.1817 call with Curran on Thompson / 9.7.1817 Thompson at Curran's / 4.11.1817 Thomson at Curran's funeral / 14.11.1817 Thompson sups = ? William Thompson DNB 1775-1833 though his DNB article implies he first came to London in 1822
10.1.1821 Thomson at Aldis's
27.10.1826 Thomson at Hodget's
25.3.1829 Thomson at Reynolds' / 17.7.1829 Thomson Luttrel at Reynolds'
27.1.1832 Thomson at University
4.7.1833 seek col Thomson 5.7.1833 call on col Thomson / 15.4.1834 col Thomson at L Stanhope's = probably Thomas Perronet Thompson DNB 1783-1869