As you are in Devon(I assume in the UK) a European shop would be the easiest way. In the UK there are several decent mail order shops, like
www.warehouseexpress.co.uk SRS Microsystems ? Pentax, Nikon and Olympus Digital Cameras, Camera Accessories and Lenses (they also have an ebay store)
and more. I myself have bought of both these shops and got good, fast and reliable service and great prices.
You can also buy off ebay, if a lens is much(!) cheaper than from a local/national dealer, but be aware, that you probably have to pay higher postage, VAT and customs duty on top of the ebay price, which, at the end of the day, may cost more, than buying locally...
But for very low prices there are a few ebay-dealers, which are reliable and so cheap, that I bought from them, despite all the import costs and saved considerably, compared to European prices:
Prodigital2000 in Canada is here:
eBay Shop - Prodigital2000: Wir verkaufen Lowepro, Canon, Digital Accessories.
or Cameta Camera in the US:
eBay Store - Cameta Camera New York Authorized Dealer of all major brands:
There are certainly a lot more dealer to recommend (like B&H or Adorama), but the lenses I suggested are in a medium price-frame and so at the end of the day, the difference between the UK and overseas is not quite as significant.
Ben