I took some patience dock home to steam it.
Removing the midribs from the leaves isn't necessary. I left the midrib on one large leaf and discovered that it cooked almost as quickly as the rest of the leaf i.e. in about three or four minutes.
The texture was pleasingly soft; I can see why it is said to make an excellent purée, but it does need some extra flavour - salt, pepper, chilli, ginger, sorrel maybe.
I think I'll be mostly adding it to salads though. It will be useful to be able to gather large mild leaves produced in abundance. There are plenty of small fiddly salad leaves for the picking at this time of year - they need some patience!
N.B. I have some patience dock plants for sale at the moment.