Antonia Forest fans

Being one of them iggerant colonial types, I don't really understand the English class system. I have seen the Marlow family described as both 'upper middle class' and 'landed gentry'. Some questions:

1. What is the difference between 'upper middle class' and 'landed gentry' in terms of typical profile, assets/income, attitudes and behaviour?

2. Seeing Trennels is a large farm entailed to the Marlow line, the family is presumably 'landed', but does this necessarily make them 'gentry'?

3. Whereabouts do senior naval officers stand in the grand scheme of social status and income?

Tell all, ye wise and knowledgeable...