Getting back to the original topic as to what batteries to buy - this is a question that we are now considering. We were fortunate to have been able to buy sets of used batteries from a forum member more than five years ago and these are still going strong. Although previously used, the batteries are very well made and have lasted well. The seller hasn't been active on the forum for some time and there are no cheap alternatives that match the specification of the originals. The original batteries were from the Delphi Freedom range - part number BU-31SP-115S - see my old post here.
My only choice is to shell out around £300 per battery for these:
The specification is close (if not identical) to the Delphi products that we purchased here for just £25 each!
Why pay £1200 for four batteries? Well they have a hard life on Alnwick and these high spec units are the only ones that seem to cope with our live-aboard conditions and the mixed charging regime and vibration that comes from our old Kelvin K3.