If you send money to someone on PayPal it is normally pretty instantaneous. But, for some reason, if you are a seller and refunding a buyer (in part or full) and your current PayPal balance is £0 (because you needed to transfer your money into your actual bank account where it is more usable!) then PayPal draws the money from your bank account through what they call as e-cheque, and this takes 7-10 days to get from your bank account to the PayPal account you are trying to refund.
It is really annoying and frustrating, both for the seller and buyer. The buyer is usually already not happy, and then you have to tell them that they can have a refund, but they have to wait 7-10 days for it. They don't understand why this is, cause when they pay for something or send money through PayPal it is instant, so they then think the seller is trying to pull one over on them.
Thankfully I haven't had to do many refunds, but I always dread if I need to do one, and my balance is at zero - it usually works out quicker to wait for the next sale to come in to give you funds, than wait the 10 days.
I did phone PayPal about this, and they said that I could send the payment as a separate transaction, and not use the refund process, but this could cause problems with the buyer saying they hadn't actually been refunded, and then raising a claim against you.