Hi Annabel,
In this case, you should allocate the £290 to a dummy invoice with the relevant supplier details, but using the account code Overpayments. If this does not exist already, you would have to create this by heading to Settings > Accounting > Account Codes > Create Account Code. The account name should be Overpayments where the account type can be any current asset type.
After this, to record the refund, you would process this by a Money In for the relevant bank account. Fill in the details and on the next page, use the Refund tab, and again, using the same Overpayments account code. Please see the example below:
Please let me know if there are any further queries.
Thanks,
Tommy