I found the story in Barbara Merz's ''Temple, Tombs and Hieroglyphs' paraphrased unfortunately.
Weigall was excavating a tomb containing a mummy and not much else when a sudden darkness made him look up and he saw clouds rolling overhead and knew he was in for one of Egypt's rare cloudbursts which was problematic because his tomb was at the bottom of a vertical shaft and would inevitably fill with water. Weigall of course was in no danger thanks to his rope but the mummy, about the only worthwhile find he'd made, would never survive the wetting. He had to get her (he'd determined it was a woman) out of there somehow. After a certain amount of experimentation he hung the mummy, which was fortunately very light, over his back with her wrists tied together under his chin. Up he climbed but before he reached the top the mummy shifted, half slipping off his back. he turned his head and found himself looking DIRECTLY into the less than charming face of his mummy now resting affectionately on his shoulder!
Apparently he neither broke his neck nor broke the mummy.