In a JCT D&B Contract there are 2 delays. One is a contractor delay and the other is a client delay. If the programme demonstrates that the client delay pushes the PC to say 1st August and then the concurrent contractor delay brings the PC to 20th August, is the contractor entitled to the EOT until the 1st August and only liable for LDs from 1st to the 20th.
Tommy
In short you answer your own question correctly assuming that the delays take place as you describe above.
if the periods of Employer and contractor delay were entirely concurrent the answer would have possibly been that the Contractor would get a full EOT based upon the prevention principle.
Regards
Mike