456 clutch hydraulic fault

Coming back from Le Mans the other weekend (much to the amusement of all and sundry at the French tolls - BMW drivers in particluar) I lost my clutch pedal - 1st halfway then completely - and had to cog it all the way home (400 miles!) despite my best efforts to bleed the clutch at the roadside.

The reservoir looked full and no evidence of fluid loss from the bellhousing or anywhere else.

Once back I removed/stripped the master cylinder assuming the seal was bypassing but could see no fault so refitted and tried to bleed the system with the benefit of workshop facillities - no fluid came through so after much scratching of heads we noticed that the front right corner of the reservoir (which feeds the cylinder) appeared empty despite the fluid levels above the max line on both sides - I blew some compressed air back through the reservoir and to my surprise bubbles emmerged from the left filler neck (furthest from the clutch feed) - I then topped up the left reservoir to the neck and very slowly the front right corner filled and I was able to bleed the system and the pedal felt normal......until I drove it back from my workshop last night and after 30 miles the pedal is dropping again as is the bite point.

Anybody got any experience and suggestions - obviously fluid leak = no fluid = no pedal = no clutch but I cannot see any evidence of a fluid leak so I am wondering whether there is something strange going on with the reservoir which has many slotted baffles in it. A complete new clutch inc concentric slave cylinder was fitted within the past 2000/18 months miles by the former owner.

Thanks

Jeremy