How do I get correct currency data to stripe.com from my CiviCRM transactions
It transpired that the error was due to the fact that the affected transactions were recurring payments, and the stripe extension did not pass currency data to stripe.com. So, in my use case, a user has submitted a recurring payment in GBP, and a subsequent user has attempted to submit the same payment in terms of amount and frequency, but in USD. As the currency was not being included in the payment plan data being submitted, stripe.com assumes that the plan is in GBP, because that is how the plan was originally defined, and so takes the payment in GBP.

Having liaised with the extension developer, this issue has since been addressed and I'm aiming to test the changes as soon as possible to see if it has resolved the issue.