Base for vi-spring bedstead mattress

I have a vi-spring classic bedstead mattress on a slatted base with mattress pad on the slats. However the slats are just within the advised gap between each to avoid the mattress sinking into the slats. I have also placed a duvet on top of the slats. However, after using the mattress for only 3 months I am finding that my backache is improving during the day-time but getting worse at night. I realise that backs stiffen at night but am wondering if the mattress is dipping a bit and therefore not providing sufficient support. It is medium tension and I weigh 9.5 stones. Do you think I need to replace the bedstead with a divan base and if so would it be better to sprung? Or should I try the MDF board method first?
Many thanks for your advice,