** British scientific instruments maker Spectris SXS.L down 3.6% at 4015p
** SXS said on Friday it had decided not to proceed with U.S. private equity firm Advent's buyout proposal
** SXS's decision comes days after backing rival suitor KKR's KKR.N $6.4 bln offer
** Stock up 66% YTD vs FTSE mid-caps' .FTMC 6.36% rise