During the colder months there is a cold draft at the bottom of the stairs and I had a new drop down loft hatch installed 18 months ago as I was convinced that was the cause. It had become warped. But the draft remains with all windows shut! The loft is fully insulated and I have put rubber insulating tape around the edges although there is a small gap in places where it has warped very slightly but I can't feel a draft. There is no heating on the landing. Is there anything flat I could stick over the loft hatch joins as a seal?
If you can't feel a leak then there probably isn't one, at least not there. It could be that your loft hatch (attic door for the US crowd) is leaking a load of heat. Is the door itself cold? Loft hatches are typically made of very thin wood, which doesn't have much in the way of insulating properties. I've always found it helps to pull a layer of insulation over the loft opening before closing the door to help prevent loss of heat.
One thing about home improvements is that sometimes when you fix something you notice something else that was covered up before. You insulate your loft and your upstairs is much warmer, so now you notice the leak that was always there but was covered up by the massive heat loss from your loft. You'll fix that leak and then notice another one, and another. Then after fixing all those leaks you'll have to re-balance your radiators because people are getting heat stroke in the study. So it may be that the loft hatch isn't the problem in which case you need to have a hunt for the real culprit. I use the back of my hand and fingers to search for leaks, which could come from badly sealed windows, light fixtures leaking air through the ceiling, a hole for a waste pipe in the bathroom, etc.