It is some years since I opened one up, and memory dims with age. However as I recall the part holding the inserts slides out of the cover. If you look towards the top of the plug as presented in your picture, there is a square hole for a retaining latch. This latch and its partner require carefully poking or prising back, to allow the inner section to slide out.
Have a good look first, and check if what I have said seems OK.