The Krobahn Tap Backnut Box Spanner (KB-MBBN2732) is a double-ended spanner made of high-grade steel designed to fit standard plastic and brass tap back nuts of 27mm and 32mm. It is available from a variety of online retailers. 
Product Details
Model Number: KB-MBBN2732
Material: Manufactured from high-grade steel.
Design: Double-ended box spanner for quick assembly and disassembly of tap back nuts in confined spaces.
Length: 210mm.
Spanner Sizes: Fits 27mm and 32mm back nuts, which are standard sizes for plastic and some brass fittings.