There's something when they're round the breakfast table where Jon and Geoff are talking about hawking with their Dad, and Jon says that Geoff was fishing quietly in the bay while "the rest of us" were rushing round the country looking for lost hawks. But he doesn't specify who "the rest of us" is - there are no more brothers, or presumably sisters - Anthony Merrick may be on the list, but it doesn't say. Or is there another reference later on?

Anyway, if Anthony is Geoff's age, and assuming that Geoff was about 22 when as a penniless lieutenant he married under-age Pam, and Giles followed quickly with the twins 8 years later, then he would be 31 when the twins were born, and 29 when Patrick was born. For what that proves. :-)

My view, for what it's worth, is that at least one of Anthony or Helena Merrick isn't very good at producing children. I suspect they would have had at least 2, for choice.