This account is from The Gentleman and London Magazine, pg. 701 of the 1778 volume, December issue.
R. Meshullam Zalman Ashkenazy was R Jacob Emden's son. He was the rabbi of the Hambro' Synagogue in London, a position he held from 1764 to 1780. His tenure was not without controversy, not the least of which was that this synagogue was itself a secessionary one; R. Meshulam Zalman's cousin, R. David Tevele Schiff was the "official" Rabbi of the Great Synagogue, and thus the Chief Rabbi of the Ashkenazim in England. Prior to that point, the Hambro' Synagogue had typically regarded the rabbi of the Great as their rabbi too.
R. Jacob enjoyed some small satisfaction in life because of his son's appointment, but apparently my version is all backward, for in his it was the other shul which seceded!
It's worth reading the following from R. Ya'akov Emden's autobiographical Megillas Sefer (pp. 209-10) :
Here's a translation in Charles Duschinsky's The rabbinate of the Great Synagogue, London, from 1756-1842 (1921) (pp. 74-75) (with some minor modifications of my own):
"In the month of Nisan of the same year (1765) my son Rabbi Meshullam Zalman was elected as Rabbi of the Hamburger congregation in London, likewise a result of my activity and endeavors for some time past, and after I nearly gave up every hope for it. For he had many opponents on the part of the Synagogue in Duke's Place (דוקספלעס), which separated from the community (!) and elected another Rabbi, R. Tevele Schiff from Frankfort-on-the-Main. It was, however, from God, and so all the plotting and obstacles, the opposition placed in my son's way, could not frustrate his election. Even after he had duly been elected they conspired against him, and people wrote me letters threatening that, if he came to London, they would attack and abuse him. All this was done at the instigation of that man "Laze", a pupil of "that man" (ie, R. Jonathan Eybeschutz) who made special efforts and wrote me letters, full of perversions and untruths, with the intention of frightening me so that I should prevent my son from accepting the position. The congregation of the Hamburg Shul , however, was anxious to have him, and they had warned me beforehand to take no notice of that shameful letter. He visited us here, and remained during the past Shavuot festival, and all the most notable men of the three Kehillahs (Hamburg, Altona, and Wandsbeck aka Ah"u) gave evidence of the respect they felt for him. He left us and entered upon his duties in London at the middle of Tammuz, and was welcomed with great honors and with joy. I have since heard that even his former enemies have now become his friends. May God grant that he rise higher and higher and be blessed with children."There is extant a printed prayer recited by R. Meshullam Zalman in December of 1776 on the occasion of a fast day declared for the success of the British soldiers fighting in America, but as of yet I haven't located it.