We were in a similar position earlier this year FrostyM. We ended up keeping two cockerels separately just as a backup to the ageing cockerel with the three hens. The idea of two was to keep each other company.
We have been in the position of having 3 cockerels and 'losing' them all. The first one was sold to a breeder who had just discovered her cockerel was infertile, the second died in a fox attack a few weeks later and the last badly injured himself (twisted leg), went onto anti-inflammatories and subsequently had a heart attack. We had to buy a replacement.
That was just bad luck Frosty and two should be fine. Problem is with just 5 hens two cockerels with them will probably fight. If you split them up into two flocks you will get over-trodden hens. So I think with limited space you will, as you say, have to pick one and hope.
You could pick one and try him out before despatching all the rest perhaps. Keep two more, as we have done. Then when the hens start to lay and the chosen cockerel is treading them you could check the blastoderm, which we have found is very noticeable after a few days of egg storage at room temperature. Darkbrowneggs has a site showing the blastoderm which should come up with a Google search.