Thanks for the reply.
I figured it out - if you remove the coloured lens, the hole in the wing can be seen to have small notches at the 3 and 9 o'clock positions.
A small screwdriver through that notch can be used to push one tab towards the centre and then with a bit of a twist it can be inserted at the other side.
In the end I was prepared to be quite brutal as I've already ordered a new pair with clear lenses....now I have some spares