Not surprising really. The submersible caravan pump came with the van and that's over 20 yrs old now... so-
bearing in mind:
a) they are not designed for other than water
b) and I've been transferring chlorine-based VWP (and sterilising pump and feed pipe) which is a practice I've not done before this season
-forgetting to rinse through afterwards with clean water after use is the probable cause of it's demise.
Conclusion is that the VWP has expanded the internal seal/s allowing ingress of solution as it worked fine on last use Sunday.
Need to keep a spare as they are still only a tenner... and rinse afterwards...