Perryman, I'm not an engineer and have no skills to sketch anything. Just did a little checking on the internets... A 50m straight ramp is against regulation, at least per that site I listed. It makes sense though, could you imagine a disabled or elderly person moving 50m up an incline with nowhere to stop for a rest? I imagine that's why they limit it to 10m at a stretch.

If that's the case, the extension is not the only problem. 50m + of ramp would meander all throughout that subway taking up all available space - leaving no room for pedestrian foot traffic, stairs and the like. This is probably why any suggestion gets shot down.

The only alternative would be a pedestrian overpass (over the rail line) which I doubt Lewisham Council would pay for. I don't think there will ever be an option except for the less able-bodied to traverse the steps or take a wheelchair around the A205 underpass.