Tightening Essex Flange

Hello All,
Does anyone know where I can buy a 1 7/8" box spanner?
I've recently purchased an Essex Flange for my hot water tank. I've drilled the hole, fitted the flange (using soft wire supplied). My problem is tightening the nut; I don't want to remove the insulation from the side of the tank just to get an adjustable spanner around it.
Before anyone asks I've not gone for a Surrey Flange because Salamander voids the pump warranty if it's mounted in the loft and you don't use an Essex Flange. Stuart Turner also recommends an Essex Flange.
Thanks in advance. Martin.

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