Does it have a DPF? My 2.4JTDm 159 used to drop to low 20's while it was regenerating. Regen should start after about 15 minutes from a cold start, and complete in about 15 mins after that. If you've been doing runs of less than 30 minutes, it may well keep trying to regen as the previous cycle didn't complete. If this is the case, I would recommend, taking it for a 45 minute drive on open roads asap; you don't want a clogged DPF because it's failed to clean properly. There's lots of parameters that have to be met before a regen will start, and then even more that will cause it to abort before completing (eg if left idling regen will abort). I'm no expert, but it could be that being left idling for 30 mins or so during the MOT has caused the DPF to be more clogged than normal, so it's vital that you allow it time to clear itself or it will be an expensive trip to the Dealer to have it manually regenerated.
Is the Info Centre definitely still showing "55 hours"? If the battery was disconnected during the garage work, it would presumably reset to zero.