OK here's the deal.
The sump nut is indeed at the back of the sump not, as I originally thought, underneath the sump. It is 17mm.
You undo it anti clockwise and if your spanner is below the sump (think 6 O'Clock on a clock face) you push towards the driver's door (in the UK!!) to undo.
Now you get a free tip which I discovered the hard way.
Unlike most filters, the seal does not simply sit on the face of the filter it actually fits very neatly into a lip on the filter. You know it is sitting properly when you wave the filter about and the seal stays in place. Note too that it is important to get the seal the right way round so have a look which way it is facing before you mess with it.
If like me you were taught to lubricate the seal lightly with oil before fitting the filter make sure the seal stays in place. I clumsily disturbed it which meant I had a nice dripping noise on the floor when I started the car.
Not life threatening but I had to take the filter off and reposition the seal before refitting.