Now they are all neutered and everything, my cats can go outside (again, since they were born outside, and Frida was stray). They won't let me pick them up so to get them registered on the microchip catflap, I've propped up the flap and set the catflap to register. Got Frida and Karl this morning:
fast forward to 1:50ish as before that Karl is just sitting looking through the flap really
Frida went out and then looked very unhappy and came back in with some encouragement - I stopped filming so I could set the catflap to record her chip when she came back in. I think she was unsure that she could come back in the same way she went out. Don't think I need a step there but clearly it's a little high. She went straight upstairs and aside from stopping on the stairs for some strokes she's been upstairs since.
Karl wary of the clicking sound but he did also go out afterwards - once just like Frida, only onto the patio right in front of the door, then straight back in. The second time he went away from the door, round to the side of the house, he looked really scared and I went out and could hear him loudly mewing and very wide eyed, trying to get under the gate (couldn't, which is good, I live in a quiet cul-de-sac but I'd still prefer them to stay away from the road) then I got him to come back round and he dived through the cat flap. Really oddly seemed like he didn't know how to get back in, even though he'd already done it and just gone out the flap.
Still need to get Rosa registered, she was watching as Frida went through. Not sure they'll get it when the flap is not propped open, but as they won't let me pick them up, I can't just pick them up and shove them through it like I did with my last cats. I'm sure they'll get there.
Bit surprised by how scared Frida and Karl seemed, given Karl was born outside. Frida maybe worried she won't be allowed back in - really want to know her backstory!