Sounds like a plain old rip-off Greg
From HMRC website
Import VAT rate
The percentage charged is the same VAT rate that applies to similar goods that are sold in the UK.
The import VAT percentage rate is applied to the total value of the goods. In the case of goods brought in this is the sterling equivalent of the price paid abroad, as shown on the receipt. In the case of goods posted from outside the EU, it is the amount on the Customs Declaration, which includes the price paid for the goods, the cost of transport, postage and packing, insurance and any duty that may be payable.
Given £39.99 included P&P, there was no customs duty and the clearance fee is VAT exempt, the Import VAT should be 20% of £39.99 or about £8.
I would definitely complain