##STR1## Compounds of formula (I) where R is C
1-15 alkyl, X is hydrogen, fluorine or chlorine, m and n are independently selected 1 or 0, W is C
1-15 alkyl or alkoxy, CN or halogen, rings A and B are independently selected from phenyl, laterally fluoro- or chloro- substituted phenyl, transcyclohexyl, pyridyl, pyrimidyl or dioxanyl. Preferred embodiments have R=C
3-8 and where W is alkyl or alkoxy then it is also preferable that W=C
3-8 . Where m=0, then preferably W=CN, and where m=1, then preferably W=alkyl.