My magnesium serum is 0.70mmol/l is this low?

Yes: The normal range at most labs is 0.85-1.10 mmol/l at most labs. 99% of our magnesium is inside our cells so if your serum level is low it means you are quite deficient. Increase dietary magnesium (dark leafy greens, seeds, beans- see http://whfoods.org/genpage.php?tname=nutrient&dbid=75 for details). Try Magnesium Taurate, Glycinate or ReMag- see https://drcarolyndean.com/magnesium_miracle/