A few points

1.  We start and finish on the Albert Embankment down the steps from Westminster Bridge.  https://w3w.co/taped.video.able.  

2.  We run towards Lambeth Bridge

3.  Go under the pedestrian tunnel

4.  Keep on the pavement towards MI6 and Vauxhall Bridge.  Careful because it gets busy with pedestrians and cyclists.  

5.  Keep M16 on your right.  Turn the corner on the pavement and go over Vauxhall Bridge.

6.  Go over the Bridge.  Once you're over the bridge, do not go down the steps but stay on the pavement and keep the new residential building on your right.

7.  Keep on the pavement towards Lambeth Bridge, past Tate Britain on the other side of the road.

8.  Stay on the pavement.  Do not go into the mini-park near Lambeth Bridge.  

9.  When you get to Lambeth Bridge, stay on the pavement, and go over the bridge.

10.  Once you're over the bridge, this is the only tricky bit.  Do not go down the main steps, but stay on the pavement and go round the corner and down the slope.  

11.  When you're nearly down the slope, you can take the (two) steps down to the walkway.  Caution - it might be slippy.

12.  Go back down the walkway, through the underpass towards Westminster Bridge.

13.  The finish is where you started.