I was under the impression that the Roberts frames were Columbus max OR.
The reason that some frames rust and some don't is not as simple as just 'quality'.
A frames resiliance to surface corrosion in air as described above is simply to do with the alloying elements position to each other and the gases in air on the periodic table. Some blends are less reactive than others.
A reactive element may be alloyed in to improve elasticity or ductility despite it leaving the frame more suceptible to corrosion.
What size is it, I may be feeling charitable. If nothing else it is a bitchin' wall ornament!
Somerset, like 'Heartbeat' with newer cars!
Retro MTB: 1992 Cannondale M1000 Deore XT M730. 1992 Raleigh Dyna Tech Quantum Deore XT.
Retro Road: 1992 Eddy Merckx Corsa Extra Telekom Shimano 600 STI. 1993 Dyna Tech Ti Campagnolo Athena 8 Ergopower.
Modern: 2004 Wilier scandium Ultegra 6600. 2014 Wilier GTR Ultegra 6800.