Autozone, O'rielly's, Advance Auto, NAPA have a wrench that is like a pair of adjustable pliers that works really well. $20 or so! Mine is black and takes the 5.0 cap of easily and tightens easy too. I see you are from England, might try Ebay or Amazon if available to you, not familiar with your parts/tool stores.

 

Here is the model version that will work on a 5.0L

Assenmacher JA 19015 Oil Filter Wrench