I tried the tuna trick and whilst they were desperate for the tuna they wouldn't go through the flap for it
By the time I went to bed last night I'd got to a point where they had both walked through it with me holding it open part way for them which is progress!
As it was dry overnight I left the flap held open with string so they could get used to being able to run in and out of the house as they wanted and that the flap was how they get in and out.
I've let the flap down again (have covered the magnet with a few layers of masking tape so it's less effort for them) and I've put some stinky sardines in the house and I'll go back out in a bit and see if that has tempted them to brave going through it. When I left them to come back inside they both had their noses pressed to the flap looking longingly at the sardines and Pinball is squeaking and squealing his head off.
Hopefully once Bilbo figures it out Pinball will quickly follow as whilst Pinball gets himself in a tizzy if anything is new or anything has been moved he idolises Bilbo and sticks to him like glue and so I *think* he'd probably follow Bilbo through a 'scary' flap just to be with him.
Part of me wonders about shutting Bilbo in the house and seeing if Pinball will go through the flap to reach him but I'm not sure if it's a bit soon to do that (don't want to traumatise him!) and whether I'd be better waiting until Bilbo is confident with taking himself through a flap on his own.