I had exactly the same problem (explained here – http://cyclonembc.editboard.com/maintenance-bike-tips-f5/disc-brake-help-t887.htm)
The lever was gradually finding it’s way to the bar, although a few pumps would get the pressure back again. Tried bleeding and replacing the fluid but it had no effect, there was no air in the system
Turned out the seal on the master cylinder piston was goosed. Instead of pushing fluid down to the brake, the fluid was coming back up into the reservoir. New seals were a couple of quid but it means dismantling the lever – not that hard really. Been absolutely fine since. This was on a Mono M4 but the culprit lever is identical.
This is the page I used to help me do the job. The master cylinder piston is the little thing next to the spring in the exploded picture with two black seals on it. I replaced both of them.
Hope Mini/Mono/Moto Lever Overhaul