Our last two kingsize beds have both been Vi-spring. The first an Earl de Luxe matress and sprung edge divan from And so to Bed (1986), and the most recent a Regal Firm Tension Divan Set from Harrods (1997).
We tried replacing the mattress (retaining the sprung base) with a John Lewis Natural Collection Linen 6000 Medium tension (on special purchase a couple of months ago), but that didn't work at all for either of us (especially my husband) .
He is 6ft tall and weighs 14 st,a side sleeper who likes to 'feel supported''. I am 5ft 6in and just under 10 st, front/side sleeper and rely on the topper to provide a touch more comfort. Whilst he would consider purchasing another Vi-Spring mattress, I baulk at the price and would be interested in which mattress you would recommend.