Natural pocket sprung mattress for a side sleeper with neck pain?

I'm looking for a King size, natural, pocket sprung mattress. Taking into consideration the following points, please could you recommend one?

Bed base: sprung slatted ottoman bed base (Furl Illusion w.steel enforced base and extra gas pressure). Base is new.
Size: King size
Sleep position: usually sleep on my side, sometimes on my back.
Weight: just over 100kg
Height: 5'9"
Health: neck and some shoulder ache (the side I sleep on)