Can anybody recommend a nice green hilly area to lookout over the city as the sun is setting, either in or near the SE23 area? Maybe in one of the parks? Or somewhere along Forest Hill?
Top of Blythe Hill fields: lovely views of Canary Wharf. You can pop in the Honor Oak for a pint after

One Tree Hill, up at the old WW1 gun platform.
How about the Horniman Park? - views of St. Pauls Church etc.
I mean St. Pauls Cathedral of course.
Each morning when I'm screaming down Forest Hill Road towards London's Glittering West End on my pushbike, scattering foxes and assorted motor vehicles, I glance up for one brief giddy moment and I think I catch the cornucopia of London sights...The Eye, St Paul's, The Angel of the North (what the?), The Gherkin et al.
This view is best caught on the road on a pushbike (edging towards supersonic speed), in the early morning (not sunset then) with a bit of mist floating in the air. Aaaaah, Love London.
You should be able to spot Big Ben, just to the left of the London Eye (which has the Post Office Tower as it's axis).