Whilst anything added (snorkel, roof rack) will affect the Defender's otherwise aerodynamic shape (sarcasm guys!), surprised it would knock that much off
I don't know whats gone wrong. I had been getting 30mpg on my 200tdi 110. I recently fitted a snorkel now its down to the 23mpg mark. I even gave it a treat and put a new air filter in it.
Adding the snorkel has very little to do with Aeros its about airflow getting to the engine. A snorkel will drastically change the air patten for an engine also ambient temperature and humidity will effect the MPG. Also you engine Oils make a difference as well thick heavy oil will draw your mpg down