Where was the product shipped from? EU / USA / Other?
Did Dell provide a complete service (ie inclusive of freight)?
Did Dell provide a full VAT invoice?
I'm sure DHL are simply following standard procedures. They will be charged by HMC&E for these sums and if the paperwork from Dell is incomplete then you look as if you are been charged for Dells errors.
You may well have to pay the invoived £13.13 and reclaim it from Dell.
However it's a minefield and I wish you luck in sorting it out. I'd be tempted to write the £13.13 off as a bad experience and not buy fron Dell again.