Costain Regulatory News (COST)

TR-1: NOTIFICATION OF MAJOR INTEREST IN SHARESi

1. Identity of the issuer or the underlying issuerof existing shares to which voting rights areattached: ii

Costain Group plc

2 Reason for the notification (please tick the appropriate box or boxes):

An acquisition or disposal of voting rights

X

An acquisition or disposal of qualifying financial instruments which may result in the acquisition of shares already issued to which voting rights are attached

An acquisition or disposal of instruments with similar economic effect to qualifying financial instruments

An event changing the breakdown of voting rights

Other (please specify):

3. Full name of person(s) subject to thenotification obligation: iii

Henderson Group plc

4. Full name of shareholder(s) (if different from 3.):iv

5. Date of the transaction and date onwhich the threshold is crossed orreached: v

31 August 2016

6. Date on which issuer notified:

1 September 2016

7. Threshold(s) that is/are crossed orreached: vi, vii

Above 5%

8. Notified details:

A: Voting rights attached to shares viii, ix

Class/type ofshares

if possible usingthe ISIN CODE

Situation previousto the triggeringtransaction

Resulting situation after the triggering transaction

NumberofShares

NumberofVotingRights

Numberof shares

Number of votingrights

% of voting rights x

Direct

Direct xi

Indirect xii

Direct

Indirect

Ordinary

Below 5%

Below 5%

6,250,500

6.07%

GB00B64NSP76

B: Qualifying Financial Instruments

Resulting situation after the triggering transaction

Type of financialinstrument

Expirationdate xiii

Exercise/Conversion Period xiv

Number of votingrights that may beacquired if theinstrument isexercised/ converted.

% of votingrights

C: Financial Instruments with similar economic effect to Qualifying Financial Instruments xv, xvi

Resulting situation after the triggering transaction

Type of financialinstrument

Exercise price

Expiration date xvii

Exercise/Conversion period xviii

Number of voting rights instrument refers to

% of voting rights xix, xx

Nominal

Delta

Total (A+B+C)

Number of voting rights

Percentage of voting rights

6,250,500

6.07%

9. Chain of controlled undertakings through which the voting rights and/or thefinancial instruments are effectively held, if applicable: xxi

See Annex 1 below.

Proxy Voting:

10. Name of the proxy holder:

11. Number of voting rights proxy holder will ceaseto hold:

12. Date on which proxy holder will cease to holdvoting rights:

13. Additional information:

This disclosure has been calculated based on total voting rights 102,813,520.

14. Contact name:

Scott Dickenson

15. Contact telephone number:

020 7818 4184

Annex 1: Information in relation to the person subject to the notification obligation (please tick the applicable box):

[ ] Person subject to the notification obligation is not controlled by any natural person or legal entity and does not control any other undertaking(s) holding directly or indirectly an interest in the (underlying) issuer.xiii

[X] Full chain of controlled undertakings through which the voting rights and/or thefinancial instruments are effectively held starting with the ultimate controlling natural person or legal entityxiv:

Namexv

% of voting rights if it equals or is higher than the notifiable threshold

% of voting rights through financial instruments if it equals or is higher than the notifiable threshold

Total of both if it equals or is higher than the notifiable threshold

Henderson Group plc, Henderson Group Holdings Asset Management Limited, HGI Asset Management Group Limited, Henderson Global Group Limited, Henderson Holdings Group Limited, HGI Group Limited, Henderson Global Investors (Holdings) Limited, AlphaGen Capital Limited

Below 5%

Below 5%

Below 5%

Henderson Group plc, Henderson Group Holdings Asset Management Limited, HGI Asset Management Group Limited, Henderson Global Group Limited, Henderson Holdings Group Limited, HGI Group Limited, Henderson Global Investors (Holdings) Limited, Henderson Global Investors Limited

Below 5%

Below 5%

5.62%