Hi David,
Take a look at this website: http://www.nutritionalmagnesium.org/
then you might like to take a look at a recent discussion thread we had back in February about pain and a possible way to manage low magnesium levels....see here http://www.cmlsupport.org.uk/node/7763
Personally I find magnesium supplementation generally very beneficial- and especially for muscle and joint pain/weakness. I agree that there is an issue with absorbing enough magnesium through the digestive system and found that the answer is to use either an 'oil' or a cream based product- see above link for where you can order this. I use 'Ancient Minerals' products as I find them very effective- if a little more expensive- and the cream based magnesium is really easy to use.
Soaking in a hot bath with the addition of Magnesium Chloride flakes might help with both cramps etc. as well as being generally relaxing- mangnesium is easily absorbed through the skin-
see here
http://www.amazon.co.uk/Magnesium-Chloride-Flakes-5kg-tub/dp/B007OX63SY/...
You could try supplementing with magnesium powder which is diluted in water/juice etc. Some people do find that has a laxative effect so you have to judge the dose accordingly.
http://www.naturesremedy.co.uk/pages/magnesium_171061-z=489337&p=54991.c...
As for Calcium- I have not found much need to supplement as you can get enough from food if you have a varied diet that includes calcium rich foods- cheese etc.- there is some research that says with low magnesium, calcium is not balanced and some health issues can arise from that imbalance.
Bit of mish mash but I hope it is helpful.
Sandy
BTW: a heat pack applied to cramping muscles gives immediate relief.