Form SCHEDULE 13D/A JANUS HENDERSON GROUP Filed by: TRIAN FUND MANAGEMENT, L.P.

Item 4 of the Schedule 13D is hereby amended and supplemented by adding the following: On June 16, 2026, the Issuer, Parent and Merger Sub entered into a side letter agreement (the "Side Letter"), which further supplements and amends limited terms of the Amended Merger Agreement (as further amended and supplemented by the Side Letter, the "Merger Agreement"). Among other things, the Side Letter provides that (i) the closing of the Merger will occur on June 30, 2026, subject to the satisfaction or waiver of all conditions to closing as set forth in the Merger Agreement (ii) conditions related to those regulatory approvals received in connection with the Merger as of the date of the Side Letter are satisfied as of the date of the Side Letter; and (iii) the date after which the Merger Agreement may be terminated if the Merger has not occurred on or prior to such date (referred to in the Merger Agreement as the Termination Date), shall be September 20, 2026. Importantly, while the Side Letter reflects an intention of the parties to effectuate closing of the Merger on June 30, 2026, it is noted that the closing remains subject to the satisfaction of all closing conditions in the Merger Agreement.

Item 5(a) is hereby amended and restated in its entirety as follows: The responses of the Reporting Persons to rows (11) through (13) of the cover pages of this Schedule 13D (including, but not limited to, footnotes to such information) are incorporated herein by reference. As of 4:00 pm, New York City time, on June 18, 2026 the Reporting Persons beneficially owned 25,654,382 Ordinary Shares, representing approximately 16.65% of the Issuer's outstanding Ordinary Shares calculated based on 154,075,608 Ordinary Shares outstanding as of May 6, 2026, as reported by the Issuer in the Form 10-Q.

Item 5(b) is hereby amended and restated in its entirety as follows: The responses of the Reporting Persons to rows (7) through (10) of the cover pages of this Statement (including, but not limited to, footnotes to such information) are incorporated herein by reference. Trian Partners AM Holdco II, Ltd. ("Trian AM Holdco") beneficially and directly owns and has sole voting power and sole dispositive power with regard to 25,654,382 Ordinary Shares, except to the extent that other Reporting Persons as described in this Item 5(b) may be deemed to have shared voting power and shared dispositive power with regard to such Ordinary Shares. Each of Trian Fund Management, L.P. ("Trian Management"), Trian Fund Management GP, LLC ("Trian Management GP"), Nelson Peltz and Peter W. May, by virtue of their relationships with Trian AM Holdco (as discussed in Item 2 above), may be deemed to have shared voting power and shared dispositive power with regard to, and therefore may be deemed to beneficially own (as that term is defined in Rule 13d-3), the Ordinary Shares that Trian AM Holdco directly and beneficially owns. Each of Trian Management, Trian Management GP, Nelson Peltz and Peter W. May disclaim beneficial ownership of such Ordinary Shares for all other purposes. Each of the Reporting Persons and Massachusetts Mutual Life Insurance Company ("MassMutual") may be deemed to be members of a "group" for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended. MassMutual has filed a separate Schedule 13D filing to report the Ordinary Shares that it may be deemed to beneficially own. The Reporting Persons expressly disclaim any beneficial ownership of Ordinary Shares held directly by MassMutual and such shares are not the subject of this Schedule 13D.

Item 5(c) is hereby amended and supplemented by adding the following at the end thereof: Except as set forth in Items 4 and 6, which information is incorporated herein by reference, and in Amendment No. 17, which information is incorporated herein by reference, there have been no new transactions by the Reporting Persons during the sixty days preceding the filing of this Amendment No. 18.

Item 5(d) is hereby amended and restated in its entirety as follows: Not applicable.

Item 5(e) is hereby amended and restated in its entirety as follows: Not applicable.

Item 6 of the Schedule 13D is hereby supplemented to incorporate by reference the information set forth in Item 4 above.

Exhibit 14 - Side Letter dated as of June 16, 2026, by and among Janus Henderson Group plc, Jupiter Company Limited and Jupiter Merger Sub Limited (incorporated herein by reference to Exhibit 2.1.1 of the Current Report on Form 8-K filed by the Issuer on June 18, 2026).