Thank you for your useful comments.
I imagine on your boat the ‘extras’ boxes got ticked a few times when it came to batteries and charging, because I see that the standard spec is for 2 x 180ah lead acid domestic batteries, as for the 321.
I did not realise that Mastervolt made batteries and looking online it is clear they get very good reviews, in line with what you have experienced.
Doing more research it seems that ‘you get what you pay for’. AGM and Gel batteries cost a lot more but give a longer life, and tend to be designed for deep cycling.
My limitation is that the battery box will only hold the 2 sized batteries I gave in my first post. Mastervolt only make a Gel battery of that size and they retail for £450 each.
I guess I will try the Varta LFD180 batteries for now and take more care of watching my battery voltage. I only have the standard voltmeter/ammeter on the switch panel.
What voltage equates to your 55% discharge rate?