hi anonymous, you seem to know already mark's parents were married and had another child (details below), but you don't sound as if you have purchased the certificates from the GRO which you should, as it will give you a great deal more data to support other searches for siblings, ancestors and thus other descendants.
You are unlikely to find where and when Angus was born without his marriage certificate as he doesn't seem to have been born in England, or at least not with Angus as his first name.
Vivian's birth...
Name: HAWLEY, Vivian A
Registration district: Hammersmith
County: London
Year of registration: 1946
Quarter of registration: Oct-Nov-Dec
Mother's maiden name: White
Volume no: 5C
Page no: 1205
Vivian and Angus's marriage...
Groom: MCGRATH, Angus J
Bride: HAWLEY, Vivian A
Registration district: Kensington
County: London
Year of registration: 1964
Quarter of registration: Jan-Feb-Mar
Volume no: 5C
Page no: 2515
There is a good chance that this is the marriage of Vivian's parents but you cannot be sure until you confirm her father from her marriage certificate...
Groom: HAWLEY, John A
Bride: WHITE, Hilda
Registration district: Fulham
County: London
Year of registration: 1943
Quarter of registration: Jan-Feb-Mar
Volume no: 1A
Page no: 515
Vivian and Angus's children's births including Mark's...
Name: MCGRATH, Steven J
Registration district: Hammersmith
County: London
Year of registration: 1964
Quarter of registration: Oct-Nov-Dec
Mother's maiden name: Hawley
Volume no: 5C
Page no: 1031
Name: MCGRATH, Mark Anthony
Registration district: Paddington
County: London
Year of registration: 1966
Quarter of registration: Apr-May-Jun
Mother's maiden name: Hawley
Volume no: 5D
Page no: 931