"We know a day like that will come again, we just hope it's not too serious when it does."

How many times have I heard friends, colleagues, politicians and even senior policemen in London voice such a sentiment over the years?

Ever since the July 2005 attacks on Tubes and buses that left 56 dead, we Londoners have suspected our luck would run out one day.

I say "luck", but that is deeply unfair to the police and intelligence services, who've foiled so many plots in the intervening years.

And yet they would be the first to admit that surveillance, infiltration, bollards and barriers, armed officers and cameras can never provide a complete guarantee of security. Not when a man is prepared to drive a vehicle along the pavement on a crowded bridge. Not when he's prepared to die in the act of killing. Brutal, crude and effective.

READ MORE: Five killed in Westminster attack

There is a cliche of the English, that of the "stiff upper lip", of "Keep Calm and Carry On". Like many supposed national characteristics, it's an absurd and inaccurate generalisation that contains a kernel of truth.

This city prides itself on having survived the Blitz and IRA attacks.

"We can't let the terrorists win ... our democracy and our tolerance is our strength" the politicians will say - sentiments with which the vast majority will concur.

But it would be disingenuous to deny that this is a shocking moment. I cycle across Westminster Bridge and fight my way through the befuddled crowds of tourists outside Westminster Tube station several times each week.

Depending on my mood (and how late I am for my next appointment), I watch with wry amusement or frustration as gaggles of French schoolchildren and honeymooning Chinese couples block the pavements as they pose for photographs beneath Big Ben.

It's a deeply familiar place, one of the most frenetic junctions in this busy, heaving city. But to the 19th century poet William Wordsworth, it was a place of contemplation. In "On Westminster Bridge" he wrote:

Ne'er saw I, never felt, a calm so deep!

The river glideth at his own sweet will

Dear God! the very houses seem asleep;

And all that mighty heart is lying still!

Consolation in poetry in these bleak times? Perhaps not, but I suspect I won't be the only Londoner to stop and pause next time I cross Westminster Bridge, to mourn for those who are dead, and to wish the injured and our own fractured society can heal.