Sounds like they've escaped from somewhere.
Don't expect the RSPCA to do anything. They're useless unless the BBC are around. You might just want to inform the local police, in case they've had a missing parrot report from anyone.
Yes, put food down for them. Try to catch them if you can, but be very careful. They're powerful birds, especially when they're frightened, which they undoubtedly will be. The larger breeds are capable of breaking your fingers.
Beyond that, there's very little you can do, I'm afraid, heartbreaking as it may be.