Hi Phil,
Movicol can create the cramping you describe and it will take some time to adjust your in system, so it's good you've started with a minimal amount. Perhaps just persist for a while longer with it at that quantity and see how it goes?? I didn't use Movicol myself but Lactulose, which worked great for me and never gave me issues. It does take a bit of time and trial and error in getting dosages and soft softeners that really work for you.
I'm now a Magnesium convert but experiment with each thing individually & gently to see what helps as the body needs to adjust to all the changes. Within a few weeks, you should be able to assess what is working for you and what isn't.....