Update.
Apparently they didn't have a way to find the issue. They said if it persists they can supply me with a USB which tracks the frequency of it happening, and then provide that evidence to Volvo for a potential replacement. We'll see - it has been fine since then...
A little worry - the chap also said that the latest upgrades to Sensus on new models means that the version I have is "becoming less supported by Volvo" - the price of progress I suppose
