I saw the woodpecker straightaway again today - as I know where to find it and I've got the 'jizz' of what it looks like. What I saw was a female great spotted woodpecker (red undertail feathers) according to my bird book. It had no red cap or red nape of head. It was virtually at the top of the dead tree trunk and I noticed that it was poking its beak continuously into the same hole. It was hard to see as it was round at the side a bit, so I think I'll try and look for it some time from the other side of the river. In fact, I've discovered I can see these trees from my flat with my stronger binoculars!