Look at people like Judi Dench, Jenni Murray, Dawn French, Fern Britton, and others who are mature, have their own individual style and pay very little attention to "fashion" specially those you think look stylish. Do not be afraid to admire what somebody else is wearing they will usually say where it is from!
Think about what you feel comfortable (and smart) in, styles, colours, shapes, etc and experiment!!
Do not close your mind to "younger" trends if they suit you but "try before you buy" with internet shopping.
Look for labels like EAST, Joules, Phase Eight, Mint Velvet, not forgetting the supermarkets (Tu may surprise you) Bon Marché and Mackays. Even M&S has its moments!
If you need to, take a (good) friend or a DD with you , but if you have your doubts about an outfit, send it back
Good luck!