Recommended mattress for lower back pain

Could you kindly recommend a mattress for me .... Middle-aged male, 6'3", 17 stone, side-sleeper, sole bed occupant, prone to lower back pain. Have always had firm mattresses - currently Sealy Ortho Millionaire, which is around 8 years old and back pain is starting to get worse.
Budget preferably £500 - £800, but could go to £1,000 if this enables a significantly better mattress. Thank you.