So here's the letter I wrote to my tutor.
Unfortunately circumstances have conspired to make it very difficult to progress M820. I am now working on contract away from home until the end of September. As a consequence of long days I'm under severe time pressure in the evenings and I'm reluctant to allocate the required hours to OU work at weekends due to family commitments. Will there be an issue do you think in re-applying in future?
In fact I formally withdrew via the OU call centre. The very sympathetic woman who dealt with my case advised that I'd get a refund-voucher of around £200 which I could use to purchase a course when I actually did have time. So looks like the Calculus of Variations is just deferred.
On the one hand I'm immensely disappointed to have to give it a miss; on the other hand M820 does demand total immersion in the text - something which my current workload and working away from home absolutely does not permit.
What really irritates me is that it's déjà vu all over again. When I first enrolled in the OU's MSc programme in the 1980s I almost immediately had to let it go due to the demands on my time: full-time job at STL, work on my Ph.D plus family responsibilities. Plus ça change, plus c'est la même chose.