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If you come down Granville Park you can go straight over onto Silk Mills Path - it isn't the dead end googlemap makes it look like, turns into brief shared use path, linking up to Conington Road. That then allows you to go over the incredible Elverson Road DLR station footbridge to get over to St Johns and link up with the current route. Pic below!
If that's too fiddly, then anything that gets you to Conington Rd then Elverson Road DLR is your friend I reckon. The footbridge is fun and will cut out Loampit Road.
Just a thought anyway, not for me to tell you how to run this!
Blackheath to Lewisham way should be done via Granville Park and Station Rd. Looks like you all had a blast!