I know it's accepted for cats to be indoor/outdoor critters in the UK, and I'm not sure how that might affect the dynamics involved... but when I brought five-month-old Pearl home to join my 18-month-old Clyde, I followed all the standard advice, and it worked like a charm...
For the first day, I kept them completely separated, though they could hear and smell each other under a door. The second day, I put Pearl in her carrier and brought her to the living room, where Clyde could see her for just a few minutes. The third day, I extended that to half an hour, and that day, there was no hissing or hostility at all.
So on the fourth day, I brought Pearl out wrapped in a blanket and just sat on the couch with her while watching TV. Clyde watched from a distance, and eventually curled up on the sofa beside me!
The fifth day, I did the same thing, and little Pearl climbed out of my arms and crept over and shyly kissed Clydie's nose... and the rest is a kitty love story.
Through all of this, I was constantly trading out bedding so they both ended up smelling alike. I made sure to keep my voice low and calm, so they didn't get alarmed. And above all else, I made sure to be especially affectionate toward Clyde, so he wouldn't feel he was being set aside in favor of another kitty.
I hope this helps! And I hope someday to visit Scotland again... you live in the most beautiful country I know.