Thanks everyone for your responses. I would have replied earlier except I spent most of the day pulling apart a room divider (silky oak) for sanding and repair. Sadly the silky oak will be painted over - too much damage where the repairs have used other than silky oak.
I have put together a couple of the ideas suggested from Charles and Neville, and attached is a plan of what I will do. The plan is a simplified view of the table I am building to give some context. Neville, I won't be putting in a rail around the legs at the top, and I suspect that I have the legs at 90 degrees to the way you were imagining them. I had thought about cutting the tenon on the leg at 15 degrees, but I only have 19mm to play with in this direction, and I didn't think it would be thick enough. Hopefully the plan will make this a bit clearer, but also note that this is a concept plan and details have deliberately left out e.g. the bottom rail for the magazine rack is shown only in one elevation and is missing in the other views.
What I have decided to do is to make the mortise about 3 times wider than the leg i.e. 60mm wide instead of 19mm, and cut it the normal 90 degree angle. Then I will cut a filler block and split it in two at a 15 degree angle, giving two blocks approx 20mm each. Screw the filler blocks into the mortise leaving a gap in the middle for the 19mm rail which will be at 15 degrees. The drawing should make this clearer in the detail component. The red are the filler blocks.
Again, thanks for everyone's assistance - it was your ideas that helped me figure it out. I will put some photos up later when the table is done. That should be soon as someone is coming tomorrow to select the slab to use.