Shares in speciality chemicals group Johnson Matthey fell back after Morgan Stanley downgraded its rating on the stock to 'equal-weight' from 'overweight.' The broker raises its price target on the firm to 1,450p from 1,400p, but sees little upside potential beyond this.It says its downgrade of Johnson Matthey implies a more negative overall stance on the UK chemicals sector.Johnson Matthey, which refines platinum and produces catalytic converters for cars, has seen its share price rally sharply this year, in line with prices for the precious metal. The shares plunged towards the end of last year as the global economic crisis intensified.