You will need to view your dad's birth cert to see who his father was.
I can see the 1947 marriage,so it is quite likely that Lilac was still officially married to John Lowrie when your dad was born. IT may not list him as the father though.
Lilac wasn't free to marry Douglas Welch intil 1963 (divorced John Lowrie or he died?)
I see she was known as Lilac Mabel and Mabel Lilac.
Although they were indexed twice there will only be one certificate.
Marriages Dec 1963
LOWRIE MABEL L to WELCH DARTFORD 5B 757
WELCH DOUGLAS C E H to LOWRIE DARTFORD 5B 757
WELCH DOUGLAS C E H to WELCH DARTFORD 5B 757
WELCH LILAC M WELCH to DARTFORD 5B 757