Vi Spring problems with base

Hi My partner and I have just ordered a ViSpring Herald Superb Kingsize mattress in firm from Furniture Village. It is due to arrive some time in mid-May. in the shop I asked whether the mattress was suitable for use with a bed frame as opposed to a divan and was that it was, however I have since found advice from ViSpring which indicates that their mattresses should not be used with slatted bases which have gaps greater that 2.5″. We have just hastily measured our gaps and find that they are actually 3″. It is a wooden frame (beech coloured, although I am not sure that it is actually beech); I think that it is a solid slatted type and has had a replacement central support bar – the first being made of thin plywood cracked and was replaced with a more substantial pine bar. My partner and I are both heavy (I am 19st and my partner is heavier again), which is why we have opted for the firm mattress and gone for something of a better quality. However, I am now somewhat concerned that we have spent a substantial amount of money on something which will be ruined by an unsuitable base. I have read through your very helpful advice and replies to questions left by others but I am not sure whether your advice about pads and boards will help with the problem which we have?

Is there anything which you would advise as a fix for this, or are we facing the prospect of having to order a divan base to go with the new mattress?

I would be grateful for any advice which you can offer before we actually get the new mattress delivered. Many thanks & best wishes, Debbie >