I can replace the door lock on the external door, which means I only have the security door handle which protrudes 46mm and the gap between doors is only 15mm.

Thanks for replying JB1
I bought the security door second-hand, as by the measurements, it fitted perfectly.
The security door sits inside a frame of 25mm x 38mm tube steel, which fits…

I installed a steel security door.
Easy enough.
Unfortunately, because the security door and front door are only 15mm apart when closed, I cannot close both doors at the same time as…