Not too knowledgeable on the Defender, but have you checked the vapor lines and canister for leaks? The purge valve might be sticking, causing a buildup of vapor at the canister, or if the canister is plugged, it could cause a buildup of vapor in the tank which eventually vents and can also cause a strong fuel odor.
1994 Range Rover County LWB - Sold for Parts
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1999 Disco II, 98k, Stock
1999 Kawasaki Ninja ZX-6E - Summer ride to work.
1994 Ford Taurus SHO - Modded Q Ship.