PayPal are covered by FSA rules, but for £5.00 you are going to need to do an awful lot of work to get any re-address there.There's no harm in trying the FSA, if you want to obtain further advice and perhaps resolve the issue.
Have you thought about trying the eBay or PayPal community forums for advice, perhaps someone there as had similar experiences.
If you have checked both your eBay and PayPal accounts, then this should show if and when, and how much was debited or credited to/from each account. If it shows that £8.45 was sent by PayPal, and only £3.45 in the eBay account, then ask eBay why you are £5.00 missing. Have eBay deducted any fees!.
Regarding the standard answer, this seems to be the normal way eBay and PayPal work, hence my suggestion about using their community forums.