I live just behind the swimming baths and have heard these guys on both the mopeds and high powered bikes all week. I believe the police arrested the kids that were riding around till 3am and seized the moped. They returned with a new bike the following night.
As for the 'grown' adults on high powered bikes they are a real danger to other road users. Around two weeks ago I was walking along Dartmouth on a Saturday midday and they were all suiting up ready to ride later on that day I saw them all outside that barbers.
Spoken to the police and they said we just have to keep emailing and calling the council. Email below.
Communitysafety@Lewisham.gov.uk