I have reached the lobby with two camera and have disposed of security man and shot the lights out but cannot manage to get down the passage to tap Dohertys computer as I set the alarm off. I am careful to watch them. I have even gone back and made sure that all bodies are hidden. Any ideas please

There are subforums for individual games. Try gamefaqs for detailed walkthroughs.

I have been on every sub forum and still no joy

Cant really say, been a while since I played. Maybe you left something out to find (I remember I had left a body which got discovered and only restarting the mission did I do it right. Anyways look here, lots of walkthroughs:
https://www.gamefaqs.com/xbox/561102-tom-clancys-splinter-cell/faqs

It's been a few years since I last finished the game but it really sounds like you left a body in the open. My guess is that you either missed a body or at least one of them isn't as well-hidden as you thought.

I played the game when it was released so I definitely don't remember the area. But I know there was a discussion back then about what trigger an alarm, especially on later stage. Apparently when you leave a certain area, the game scanned the area to see something that it flags as an alarm trigger. It might be a body that's not well hidden. I remember I ended up dragging every unconscious bodes to a very dark and well hidden corner and finally pass through an area without sounding the alarm. That area might be the one you are playing now

Probably this, the game apparently scans the area for bodies left in light once you leave that area.
Speaking of which, I really wish for Chaos Theory (the non-linear one that does away with this, the peak of the series) to come to GoG.

If a random alarm goes off it means you left a body out in the open and not in the shadow.