I've now fitted an intermeidate so that all the wires have somewhere to go
Sorry; couldn't resist that.
You shouldn't have listened to a wholesaler whose first priority was to sell you a switch!
So, you once had one red and one black wire going to the outside light. One of those would be a neutral; probably the black one but, from what we know of the old wiring so far, nothing would surprize me!
The other one should have been switched live for the light - but switched from where? Did that old light ever have a switch?
The remaining two wires (one red, one blacK) were, hopefully, the live feed and switched live for your kitchen light. We know you've got live power available in there, as well as a connection to your light, because you've had the thing lit. Your problem is therefore a fairly simple one (compared to some of the multi-switch corkers that come up on here): How do you identify two wires out of four?
You could just work your way through all six possible combinations but I don't recommend that because at least one of them will trip your breaker. A meter will help but you have to know how to interpret the readings. When you have multiple wires in close proximity, capacitive coupling can throw up all sorts of misleading results. My solution would be to use a light bulb - in a holder of course!
Select each of the six possible pairs of wires in turn and connect them across the bulb. Isolate the other two in choc-block. Now observe carefully what happens when you put power on. If the test bulb lights up bright, you have a live feed and a neutral. Now connect one of them (the red one?) and one other wire (a black one?) to the test bulb and try again. What you are looking for is a partially lit bulb and, at the same time, a partially lit kitchen light. Those are the two wires that you connect to the switch. DO NOT connect any other wires to the switch.
PS: If you want to use your nice new intermediate switch, use only two of its four terminals: one from each end.
PPS: If the old outside light never had a switch, you might find not one but two live feeds in there; so which one do you use?
If they are both fed from the same breaker it doesn't matter too much but if they aren't ---