Here, I presume?
https://maps.app.goo.gl/EgCESgqVuv5eabRP9As Tincombe says, I see no signs or markings indicating a M/c bay? Hoever, as you say and GSV confirms motor cycles have parked here for years without enforcement, this gives rise to the defence of 'legitimate expectation'. BUT this can only be used once, as PCNs make you aware that enforcement is now happening.
Once you have put bike away somewhere safe, write a draft challenge as to why you think the PCNs should be cancelled, and post the draft here for comment before submitting to Southwark.
You should also be contacting Southwark for details of any other outstanding PCNs against your Reg.Mark.
I notice that in GSV (taken in 2022) there is a sign from London Parking Solutions (APCOA). Is it still there? If so, can you get a close-up? What is the relationship between Southwark and APCOA?