I didn't cross the road to check—it was still quite busy on the Portway and would have been hard to dash across. However, I turned out to be right on later walks; this is the hard-to-spot place where you can cross the road and walk beside wall you can see to the concealed entrace to Portnalls Number One Railway Tunnel.
You can barely make it out in the picture unless you know it's there but theres actually some decaying old steps and a handrail to help you up there from the kerb of the portway, roughly a fifth of the way into the photo from the left-hand side.
GPS Coordinates:
51.45934, -2.62993
Location: Sneyd Park and the Downs