My wall is over 25mm (1 inch) thick, what kind of screw/fastener do I use?
You almost certainly have plaster over "rock lath" which is a sort of early drywall board with holes in it for plaster to key into. Rock lathed plaster was common during that time--used as a time saving alternative to wood lath, but before taped and mudded drywall became popular. The board was 1/2", and the plaster another 1/2" or so. It's a very robust finish system for a wood-framed wall. Just find a stud and use longer screws.