A beekeeping friend came over and helped me locate the queen! So, she is now marked which gives me a bit of confidence to think again about the unite. Surprisingly the hive was not as weak as it was a week ago so am inclined to give it a second chance.
The swarm from this hive about 6 weeks ago is going great guns - neither hive is preparing to swarm again in the near future. These were the two hive I intended to unite.
Good decision.