Hey Jem, okay so to get the cat to go outside… to be honest they normally get the hang of it quickly if you have others that go outside, because they copy what the other cats do. But some things to try – put the litter tray in the flower bed (or other suitable cat toilet).
You then have a choice, first is leave it there to get full up (yuck) and hopefully they should move into the close by flower bed.
Or, let them use the tray there for a few days, then remove the tray but put a load of the cat litter in the flower bed, hopefully they should keep going there and over time, the cat litter will just get buried in the flower bed.
You will prob have to take away the inside one for a bit while you do this, otherwise they might keep going back to it.

On a side note, it might be worth you thinking about keeping a litter tray in the house (even if you just put it down at night so they don’t use it during the day), for two reasons, firstly, the cats might get uncomfortable holding their toilet all night (assuming you lock them in at night) and secondly if they always poo/wee outside, you might miss an early warning sign of illness, such as if there is blood in their poo.

Your very welcome. he he