I've now a very informative and helpful reply from a chap called Ken at Paypal HQ. I'll pass it on as I think it's useful for others: With a basic authorisation, your payment will show as 'Pending' in your PayPal account until the merchant captures it. A capture is when funds move from your PayPal account to a merchant's PayPal account. After the capture the transaction will show as 'Complete' in your account and you will be notified in an email from PayPal that your payment is captured.Merchants can capture the payment straight away; however in some instances they may need to capture the payment at a later date. For example they may need to confirm item availability, assemble your goods or account for late changes to the order.
In other words, PropShop have not yet taken the money from the account because they don't currently have any of the right size props in stock. It has, however, been identified in my account as money which will be transferred once PropShop send the props and take payment. It's thus been taken into account in calculating the balance of the account available for further purchases. I've suggested to Ken that this explanation of "PayPal has placed a temporary hold while we're reviewing it" might appear in their standard E-Mail reply and also as part of the FAQs on their website.
Dave M