Sorry, I might be shooting bb gun atm, but it might be Roberts Cycles frame.
From wikipedia:
Roberts Cycles was a custom bicycle manufacturer in Selhurst near Croydon, South London, England.
The shop closed at the end of May 2015.
Fastback stays and shot-in seat stays started to overtake the engraved chunky version stays with a single R. Top eyes, bottom brackets and fork crowns were now all available with ‘Roberts’ engraved in full.

But, I might be wrong.