The 
Krobahn Tap Backnut Box Spanner (KB-MBBN2531) is a double-ended, high-grade steel spanner designed for the quick assembly or removal of tap back nuts on basins and baths, particularly those that are smaller brass replacements. 
Product Details
Material: Manufactured from high-grade steel.
Design: Double-ended box spanner design for ease of use in confined spaces.
Length: 210mm.
Spanner Sizes: Fits 25.4mm and 31.5mm back nuts.