If the payment was greater than the invoice, it’ll be allocated to the client’s account rather than an invoice itself.
If you view the client (either click their name at the top of the invoice page, or Sales >> View all clients >> Select the client).
Any money sat on their account is shown as a ‘prepayment’ in the top right corner:
If the invoice itself should be £800, you would first need to amend it to show the total of £800, and then log the payment against the invoice. If you’ve already done this (I’m assuming you have as it shows £601 has been paid), then you click the ‘Log Payment’ button on the invoice preview screen, and log it from their credit:
