Hi there Simon
mate I'm going to have to disagree with Andy. This is where we get into the sit-in fans and the sit-on fans. For me, its sit-in for sure.
I went through what you are doing now about 12 months ago. Like you I am a bit of a photographer, not a fan of going out into the open water, love flat water river systems and take my wife out on shorter 1 or 2 hour trips.
So I bought a couple of plastic 2.8 metre kayaks. These have been a perfect introduction to kayaking, but I now need something longer and better through the water.
The problem with my current kayak as far as photography is concerned, is that when you stop paddling, because they are so short, they twist in the water. The longer the kayak, the straighter it will stay. If you are shooting birds or wildlife and you need to glide quietly toward your subject, you will definately need something longer and preferably with a rudder or skeg. Something I had to find out the hard way. Also, anything under about 60cm in width can be a bit tippy. So try and keep over that, 65cm is ideal.
So as we sound similar in what we are looking for, I would recommend you look for a hardy, plastic moulded, 3.5 to 4 metre sit-in kayak, with rudder. Some even have a handy little storage hold right in front of you for your camera.
Good luck mate