Derwent Macdee Metro, diaphram return spring?

I need a diaphram return spring for my Derwent Macdee Metro toilet flush. CME Sanitary Systems, who make them, only sell boxes of 12 for £12. They say all the Metro springs are the same (CME part no. DSY1400). Does anyone know where singles can be obtained? It's not the £12 that upsets me, it's the thought of having 11 spare springs rusting in the shed for ever.