First, many thanks for all your advice, it is much appreciated. I have 6 pairs for my 24v system. The pairs and voltages, after being fully charged (i.e. drawing 1A) and left disconnected for 12 hours, are as follows:
12.7 12.6 12.5 12.5 12.6 12.6 12.5 12.6 12.5 12.6 12.7 12.3
(I did that in two separate lots of 6 each so as to maintain power to the boat)
I'm afraid I don't quite understand what you mean balancing out the voltages. What do you think I should do with the one at 12.3v? Get rid of it, or pair it up with something else?
Understood about 3-4 year expected life although I was hoping it may be a bit longer!