You said you had 2 seats, one of which is the BV - what's the other? does it puzzle with the BV in the middle? I'm sorry that I can't be of more help... I'm not sure what the Navigator is... Have you considered a Frontier/Nautilus? - those seem smaller than many convertibles
ETA - I've seen pics of a Silver Cross Navigator, but never with a harness - is that the one you mean?