UPDATE 1-Johnson Matthey H1 pretax falls on auto slowdown

* Underlying pretax profit 114.4 mln stg vs 144.9 mln * Revenue declines 18 pct to 3.6 bln stg * Dividend unchanged at 11.1 pence (Adds details) LONDON, Nov 25 (Reuters) - British platinum refiner Johnson Matthey (JMAT.L) said its first-half pretax