Suitable mattress for lower back ache

Please can you suggest a suitable mattress -
I am 67 years of age (male), 5 ft 9" tall, weigh 11 st 12 lbs and have a long term stiff and achy lower back. I sleep mainly on my side but also sometimes on my back. I have a divan hard bed base. Price range (for a single size) up to about £600.