Yes I agree. Magnesium is an excellent substitute for any stool softeners out there. In my case, the transition from Miralax to Magnesium was gradual. I took a supplement of 250 mg with the main ingredient listed as Magnesium Oxide, once a day before going to bed. I think in your case, it would be easy if you continue taking your AM Movicol sachet and substitute your PM dose with Magnesium.
I had hard time choosing the right form of Magnesium. I learnt that selecting Magnesium in its vitamin form can be very confusing. So, here's some information that would help you to know the different forms of Magnesium available in the market. The excerpt is from the following website:
http://articles.mercola.com/sites/articles/archive/2012/12/17/magnesium-benefits.aspxMagnesium glycinate is a chelated form of magnesium that tends to provide the highest levels of absorption and bioavailability and is typically considered ideal for those who are trying to correct a deficiency
Magnesium chloride / Magnesium lactate contain only 12 percent magnesium, but has better absorption than others, such as magnesium oxide, which contains five times more magnesium
Magnesium carbonate, which has antacid properties,contains 45 percent magnesium
Magnesium citrate is magnesium with citric acid, has laxative properties
Magnesium oxide is a non-chelated type of magnesium, bound to an organic acid or a fatty acid. Contains 60 percent magnesium, and has stool softening properties
Magnesium sulfate / Magnesium hydroxide (milk of magnesia) are typically used as a laxative. Be aware that it's easy to overdose on these, so ONLY take as directed
Magnesium taurate contains a combination of magnesium and taurine, an amino acid. Together, they tend to provide a calming effect on your body and mind
Magnesium threonate is a newer, emerging type of magnesium supplement that appears promising, primarily due to its superior ability to penetrate the mitochondrial membrane, and may be the best magnesium supplement on the market
Good Luck !