UPDATE
I have now seen several Crafters and Sprinters with the magnets, or the whole assembly missing! Looked at replacing them and there are loads for sale on eBay and OEM ones available as well (they look easy to replace - screw on/off). So a common problem by the look of things.
I have had a look at how these are assembled - poor design. There are three magnets in each catch they are held in place by a central plastic strip and a lip round the inside of the housing. I am assuming that in the hot weather everything has expanded and the lip is not sufficiently deep enough to hold the magnets in place and the central plastic strip has torn off.
Option 1 - Go to the dealer, who says they epoxy the magnets in place. (Again suggests a common issue)
Option 2 - Replace them.
If I replace them then I have to remove the whole assembly, which I didn't want to do as the assembly seals against the outer skin of the van with the risk of not sealing and getting a water leak, plus the magnets may pull out again in the future.
After a bit of practice I found the best method of getting the magnets back in and correctly positioned, not easy as trying to position three magnets in the correct position is like trying to juggle soot.
So once I had got the technique polished I decided to epoxy them back together rather than replace them.
Job done in around 10 minutes, fingers crossed.
