Just to update all those who gave me advice on extra wide and comfy walking boots - last Saturday I went out on a boot mission and finally found some at "Go Outdoors" in Saltney, nr. Chester. The staff there were extremely helpful and knowledgeable and I tried many pairs on, finally settling on a brand called "Hi Gear" which is their own brand. They were very comfortable and had the width across the widest part of my foot that I was looking for.
They are not leather boots (I find these have no give which I need for comfort) but are waterproof.
As a plus, we joined their discount club (£5 a year) and got 70% off the boots!!
So, yesterday was my first outing in them - my country walk of 3-1/2 miles and they were very comfy and the culprit callous that had caused me so much pain was fine, no problem at all.
Can heartily recommend this store if you have one near you. Thanks to everyone for the advice and hints.