By Mark Kleinman Mark Kleinman is Sky News’ City Editor and is the man who gets the City talking in his weekly City A.M. column. This week he tackles Reaction Engines, PRS REIT and EMK Capital Concorde successor’s failure risks chain ReactionWhat about this for an adverse Reaction? Two leading City fund managers – Artemis and Schroders – have slashed their valuations of Reaction Engines, a hypersonic aviation pioneer. Their moves reflect the precarious nature of the Oxfordshire-based company’s balance sheet. Having raised £40m in a funding round last year, and £150m in total, its cash-burn rat…