Allan (or Alan) did a bit of travelling too, probably with his parents.
1910 to Australia, 1923 to China, 1925 to Algeria, 1926 to Gibraltar,
Noting that his parents only married in 1908, I feel some records that suggest Allan was born in 1907, may be incorrect. There is an entry regarding a birth which MAY HAVE BEEN his registered in Marylebone district in 1910, which is only a short distance from the Strand where his parents married. A note of his death is equally vague for my record has it that he died as a British National in France between 1926 and 1930.
As you say, Alistaire Andrew is harder to identify in public records, but assuming he may have travelled with his mother when only an infant, tracing her movements could be informative.
Algeria 1925, Egypt 1929, Kenya 1934, Australia 1935.
According to A Monro (Families in Southern Patagonia) she died in Worthing in 1976.
It is also highly probable that Alistaire was being educated in a residential school during some of those travelling times. As you say, his death is recorded in Worthing in 1988.
FreeBMD gives Alistaire's marriage in Worthing in 1940 to Maude **** and their daughter was born in Worthing in 1944. However, Rootshat policy obliges me not to reveal more details about those who may be still living. I nevertheless feel sure that there may be enough info in this for you to personally investigate further, so I hope this info helps you find out more.
Regards Roy G