Mattress for slatted base bed with 8cm gaps

I have a slatted base bed (bought from John Lewis a few years ago) with 8cm gaps between slats. A new John Lewis pocket spring mattress has developed soft spots and hard ridges/bumps after only 3 months with use on this base. Will any of your mattresses be more suitable and fare better, and will they still be covered by warranty if I buy one to use with this bed?