Raw, the leaves were acceptable, bland, and slightly acidic.
I chopped the leaves into approx. 7mm strips and boiled them in water in a small saucepan on a hob for about 5 minutes.
The resulting liquid was green, pungent and reminiscent of stinging nettle juice (these two plants are often considered to have some sort of affinity, growing in similar habitats. The dock is considered in folklore to be the antidote to the nettle). The liquid tasted acidic, bitter and dangerous. Adding dock to a sauce would make the sauce unpleasantly bitter and acid, and possibly introduce harmful chemicals (I do not know if there is anything harmful in dock) so I recommend firmly against doing this.
After pressing the cooking juice out of the leaves, they were tasted and found to be bland and unremarkable. All traces of acidity were removed to the taste: in fact, all traces of taste were removed entirely. The structure and thickness of the leaf had also been destroyed, and the remnants brought to mind thin seaweed in terms of appearance and texture. These leaves could be used as a spinach substitute, but as it is likely that most of the vitamins have gone down the sink and due to the availability of many tastier alternatives in the English countryside (not least of which nettles), I do not think dock worthwhile eating.
On final comment, the famous Lancashire dish of "dock pudding" is made using bistort, Persicaria bistorta, which, whilst closely related to the true docks, is a different plant.