UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                   FORM 6-K

                        REPORT OF FOREIGN PRIVATE ISSUER
                       PURSUANT TO RULE 13a-16 OR 15d-16
                     UNDER THE SECURITIES EXCHANGE ACT OF 1934




                       For the month of December, 2005

                                 SPIRENT plc
     _____________________________________________________________________
                (Translation of registrant's name into English)

  Spirent House, Crawley Business Quarter, Fleming Way, Crawley, West Sussex
                                RH10 9QL, UK.
     _____________________________________________________________________
                    (Address of principal executive offices)

 
Indicate by check mark whether the registrant files or will file annual reports 
under cover Form 20-F or Form 40-F.

                         Form 20-F    X         Form 40-F.....

Indicate by check mark whether the registrant by furnishing the information 
contained in this Form is also thereby furnishing the information to the 
Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

                               Yes .....        No     X

If "Yes" is marked, indicate below the file number assigned to the registrant 
in connection with Rule 12g3-2(b): 82- ________ 


SPIRENT PLC - NOTIFICATION OF DEEMED INTERESTS BY DIRECTORS, PERSONS DISCHARGING
MANAGERIAL RESPONSIBILITY OR CONNECTED PERSONS


Spirent Sharesave Trust Limited, the trustee for the Spirent Sharesave Trust ("
the Trust"), has today transferred 60,385 Ordinary shares in Spirent plc (the "
Company") at an option price of 41 pence per share to satisfy the exercise of
options under the Company's Savings Related Share Option Scheme. Following this
transfer the total number of Ordinary shares held by the Trust is :


3,266,142 (representing 0.3382% of the issued share capital of the Company).


The Company's executive directors are potential beneficiaries of the Trust
(together with all other UK employee share option holders) and are, therefore,
treated under the Companies Act 1985 as having a deemed interest in the total
number of shares held by the Trust. As a result, the directors' deemed interest
in the Trust has been reduced by the above number of shares transferred.


For the avoidance of doubt, the Company confirms that this notification does not
relate to an actual dealing in securities by the executive directors whose
interests in the Company's securities remain unchanged notwithstanding the above
events.


END




                                   Signatures

Pursuant to the requirements of the Securities Exchange Act of 1934, the 
registrant has duly caused this report to be signed on its behalf by the 
undersigned, thereunto duly authorized.

                                               ______LUKE THOMAS______

                                                     (Registrant)  
 
Date  20 December, 2005                       By   ____/s/ Luke Thomas____

                                                    (Signature)*