Thanks guys.
It’s still happening – I now understand why.
When I click any donate button, it automatically changes that amount to the amount coded into the last button on the page.
So when I click £5 on this page, it actually takes £500: http://brixton.impacthub.net/donate/
And when I click £5 on this test page, it actually takes £10: http://brixton.impacthub.net/donate-2/
Any help much appreciated