Costain Regulatory News (COST)

NOTIFICATION OF MAJOR HOLDINGS (to be sent to the relevant issuer and to the FCA in Microsoft Word format if possible)i

1a. Identity of the issuer or the underlying issuer of existing shares to which voting rights are attachedii:

Costain Group PLC

1b. Please indicate if the issuer is a non-UK issuer (please mark with an "X" if appropriate)

Non-UK issuer

2. Reason for the notification (please mark the appropriate box or boxes with an "X")

An acquisition or disposal of voting rights

X

An acquisition or disposal of financial instruments

An event changing the breakdown of voting rights

Other (please specify)iii:

3. Details of person subject to the notification obligationiv

Name

J O Hambro Capital Management Limited

City and country of registered office (if applicable)

London, UK

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

Name

City and country of registered office (if applicable)

5. Date on which the threshold was crossed or reachedvi:

20/01/2021

6. Date on which issuer notified (DD/MM/YYYY):

21/01/2021

7. Total positions of person(s) subject to the notification obligation

% of voting rights attached to shares (total of 8. A)

% of voting rights through financial instruments(total of 8.B 1 + 8.B 2)

Total of both in % (8.A + 8.B)

Total number of voting rights held in issuervii

Resulting situation on the date on which threshold was crossed or reached

9.91%

0.00%

9.91%

274,949,741.00

Position of previous notification (if

applicable)

10.10%

0.83%

10.93%

8. Notified details of the resulting situation on the date on which the threshold was crossed or reachedviii

A: Voting rights attached to shares

Class/type ofshares

ISIN code (if possible)

Number of voting rightsix

% of voting rights

Direct

(DTR5.1)

Indirect

(DTR5.2.1)

Direct

(DTR5.1)

Indirect

(DTR5.2.1)

GB00B64NSP76

27,250,190.00

9.91%

SUBTOTAL 8. A

27,250,190.00

9.91%

B 1: Financial Instruments according to DTR5.3.1R (1) (a)

Type of financial instrument

Expirationdatex

Exercise/Conversion Periodxi

Number of voting rights that may be acquired if the instrument is

exercised/converted.

% of voting rights

SUBTOTAL 8. B 1

B 2: Financial Instruments with similar economic effect according to DTR5.3.1R (1) (b)

Type of financial instrument

Expirationdatex

Exercise/Conversion Period xi

Physical or cash

settlementxii

Number of voting rights

% of voting rights

SUBTOTAL 8.B.2

9. Information in relation to the person subject to the notification obligation (please mark the

applicable box with an "X")

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) issuerxiii

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 (please add additional rows as necessary)

X

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

J O Hambro Capital Management Limited

9.91

0.00

9.91

10. In case of proxy voting, please identify:

Name of the proxy holder

The number and % of voting rights held

The date until which the voting rights will be held

11. Additional informationxvi

Place of completion

London

Date of completion

21/01/2021