Why after flushing the toilet pan does the level of water then start to drop over a period of approx 1 hour. It doesn't drop below the level of the waste pipe. There are no signs of leaks, (ceiling below sound) floor in bathroom is also dry. Toilet is on 1st floor. This has only recently started to happen.

My quess is that your vent is partially, near completely blocked. The siphon action of the water going down the drain creates a slight vacuum and pulls more water. Can you rod your vent from the roof line? I'd start there.