Style advice for 51 year old guy please?

I'm a 51 year old uk guy and have dressed in jeans and t shirts for ever and want a change.I don't want to dress like an old man and I certainly don't want to look like mutton dressed as lamb.Can anyone recommend any clothing co's or style web sites that can give me some ideas? its a case of m&s is to old and next is to young any help would be gratefully recieved