Your household insurance may not cover you if it happened outside the flat. Are you sure it didn't happen inside the flat? This guy may suddenly rememebr it happened inside the flat if he has more chance of getting some money.
But how horrible of him to sue. If he's signing on its not like he had loss of earnings.
Don't worry, even if your insurance doesn't cover it, as the accident occurred in the "common parts" of the building ie the corridor, it may be the management company's responsibility. They will undoubtedly have public liability insurance.
You will need to pass the letter of claim on to your insurance company - preferably unanswered. But do point out where the accident occurred and leave them to fight it out.
Don't panic, last summer we received a letter of claim for injury caused by someone walking into wisteria overhanging our property! Our insurance company defended the claim and the claimant soon backed down.