Well guys, I'm a couple of rides in so I said I'd report back.
Did they make a difference?

Well ride one I thought so. No close passes and I felt more comfortable knowing I had it on. Ride 2....meh, pretty similar to my normal commute with one extremely tight over take by a bus. This made me think perhaps ride 1 was more psychological.
Next few rides were more of the same.

Some other points:
It doesn't really catch my eye whilst riding, which I thought it might.
It also adds a smidgin of side visibility as the lens come button is domed.
Also it's survived a torrential downpour and doesn't seem to have let any water in.

Conclusion is it's staying. That totals me at 4 rear lights, which might be overkill, but better too much than not enough.
For £1.75 it doesn't matter how long it lasts. I'll report back though