I have been reading the replies to Narwhal with interest, and note that calcium carbonate is not a good idea.
I avoid any supplements generally, but I have wondered about taking a little calcium, vitamin D, and Vitamin K. I have just looked at the ones for sale on the Boots website, and found that the one called "Calcium + Vitamins D&K" is labelled as lactose free but not as gluten free - it contains maltodextrin. It also contains hydroxypropylmethylcellulose, which I can't cope with. The plain Vitamin D contains maltodextrin too, and says it is lactose free.
So - my question is - where to buy a suitable supplement?