Simon - It's rough trying to keep extended families together and once you've established this as a 'must have' in your lives then your more or less stuck. GO TO CANADA! The kids will enjoy a vacation.
We're 2,000+ miles from my in-laws (they're in S. TX) and 2 1/2 hours from my family. Prior to this I've been 3,000+ miles from my family for 6 years and it was always so strange to use up all my leave for visits 'home'. Now wer're looking for other jobs which can move us anywhere from Korea to Hawaii. After that move we might start looking into a final, non-mobile position and living arrangement either way it'll be near family.
Websites, snail mail, videos always help build the bridge between the months and if you children are old enough, make it a family project. I wish everyone could have their grandparents live down the street and their Aunts/Uncles right around the corner. Sure you couldn't get away with anything, but you knew where to go when thing got bad.