Quiet day was it?
Must have been!

What is it in the peaks, literally every few seconds you're spinning round from one viewpoint to another? Apologies to anyone who has that song in their head, you know the one that goes "You spin me right round baby right round"
London Bridge is THE best place for spotting, just you need to move very fast with the pen and be prepared to stare at the notes afterwards and go "Eh?! What was that?!" for ages on the way home if you're there in the evening peak
St Johns looks good to me, although not tried it. Petts Wood is pretty decent, better than Orpington in my mind as you've not got quite so many platforms to move around to find everything. Quite, camping out at the end of the slow lines platforms should get you everything easily. And it's usually pretty busy, even off-peak.
For someone slightly outside the box, you could try Rainham on the c2c network. There's some great views of the HS1 stuff from the footbridge next to the station, the best view I can find right now on Flickr is this one (I'd embed it but it won't let me) and it's a view that makes me want to visit right now for some awesome 373 action:
https://flic.kr/p/793QE7
Oakleigh Park on the ECML is great from the footbridge just north of the platforms there, some decent views can be got there. I'd link to one of my own photos but I can't find it right now. However take it from me it's well worth a visit.
Back on the SET network (why specifically the SET network out of curiosity?), I think you've got the most superior places already listed by everyone now. As for other good places around the London Travelcard zones, may I present you with the places I spent a LOT of time at in the heyday of my spotting career:
Clapham Junction
London Bridge
Stratford
Willesden Junction
Harrow and Wealdstone
Ealing Broadway
Acton Main Line
Finsbury Park
Bethnal Green
East Croydon
Wimbledon
All worth spending some time at and I got so very much by spotting at these stations. Willesden Junction in particular has sent me on a nostalgia trip, same for Acton Main Line for spotting 59s, 60s and 66s on Acton Yard as well as part of moves to see what was on Old Oak Common way back when the EWS depot was still there. Ahh, memories...