TLC sell two gang intermediates, or at least they did when I bought one.
To be clear, there are five separate wires going to the one switch downstairs: presumably one cable containing a black and a red, and a second cable containing red, blue and yellow. (Both cables should also have a bare earth, which should be sleeved in yellow/green and be connected together, to a terminal on the backing box if there is one.)
Presumably there are some completely separate cable/cables going to the other switch which does downstairs only?
Presumably there are no other switches anywhere which work any of the landing lights?
presumably a cable containing red, blue and yellow appears upstairs and connects to the single switch there?
OK, then someone may have used a four terminal switch downstairs, and it can be replaced by 3 terminal switches. Please advise which cables downstairs are connected to each other. I would guess that one of the four switch terminals is not used, and the other three are. Then read the unused one as the misssing one on the three terminal switch.
I would guess that the paired red and black each share a connection with one of the red blue and yellow. You will need to keep the ones which are now together, still together when you swap switches. The connections with two wires will transfer to L1 and L2. The single wire will go to common.
Unless it was not wired the way I described to begin with.