British citizen "A" has child "B", born in Switzerland to Swiss mother. "B" never lives in England but is a British Citizen (by descent) by virtue of parent "A". "B" has child "C". "C" can still inherit British Citizenship, *if* prior to C's 18th birthday, "B" resides in the UK for 3 years. At that point "B" is no longer a citizen "By Descent" and therefore "C" can inherit the citizenship. Even if "C" does not reside in the UK, s/he is at that point a "British Citizen (by descent)." If "C" resided in the UK for the three years as well, with "B", then s/he is then just plain British, and his/her kids would be as well (albeit, "by descent" if they were born/raised outside of the UK).
So Sandgrounder, there were two ways your son could have ended up British.
Caz - I would ask the consulate or embassy if the births need to be registered. Since the kids already have passports, there may be no need. I know when we had questions the British embassy in the US was very helpful in clarifying things (hence my understanding of "A", "B" and "C" above
). And Caz, your grandkids (childrens' children) *can* be British, if at any time before their kids are 18, your kids are resident in the UK for 3 years (as the law currently stands).