Dodgy Dual Flush

[zloog]

Morning all. Our downstairs loo is causing some grief. It is a Gala Motropole. When you use the large flush option the plunger seal does not drop back down when the water has run out of the cistern without a tap from a finger. EVen when the drop down plunger has dropped it is not sealing properly and water is overflowing into the loo. I have had it in pieces and I think the mechanism and the rubber are both knackered.

I was wondering if anyone knew where I could get a replacement for it? Are they just standard fittings for hole diameter at the bottom and height?

Many thanks

Sam