It's a long story. However, I'll supply the bare essentials:
1) Applied for F visa in a province. Application rejected, instead given 10 day * visa backdated to application date, so had 6 days left to exit China.
2) Passport thereafter had 1 page left to accommodate a new visa.
3) Trained it to Hohhot. Got fast Mongolian visa there, bussed it to Erlianhot. Left China by rail on last day of my visa validity, crossed border around 1945 hrs that evening.
4) Applied for temp passport coz I could get it fast, as opposed to full passport. Corners cut on my old passport by the ambassador. This meant said passport couldn't be used for crossing borders, but ok for Mongolia stay.
5) Visited Chinese visa office in UB to get Chinese visa in temp passport. Had marriage certificate, got visa.
6) Trained from UB to Erlianhot, but hauled off the train by Mongolian police at Jaminude, last Mongolian stop before the border.
7) Had to buy Mongolian exit visa there, as my previous Mongolian visa was in the old passport, but that was invalidated for crossing borders by the cut corners.
8) Re-entered China, 30 day L effective from re-entry date. returned to Hohhot - I had spent 6 days in Mongolia.