The Trennels that the 20thc Marlows live in was built by an ancestor who made a pile in the slave trade, and the older Trennels house was the farmhouse where the Tranters, and later the Dodds, live, isn't it? So the money that raised them from yeomen to gentry or gentry-adjacent was far more recent than the source of the Merrick wealth (presumably land?) But yes, much on a level by the mid-20thc, enough for Nicola to be only mildly struck by the house's magnificence when she's first inside it.
I always note the rather feudal bit where Mrs Bertie and Doris and lot of other women from the village go to 'lend a hand at the Merricks' do' aka the Twelfth Night ball, and we see Doris coming with the Marlows' wraps and coats during the geese scene, so Doris and Mrs B, who are offered a lift home by Mrs Marlow, stay to the very end, after all guests other than the Marlows have gone home and Mrs B is up early enough to cook all the hunters a huge breakfast in the morning.
Helena Merrick says something, apparently more to Pam Marlow than to Doris and Mrs Bertie, because PM has just offered to drive them home, like 'They've been the greatest possible help -- I'm everlastingly grateful'.
I mean, why? If large numbers of village women do this every year, can the Merricks have been paying them all? If not, why?
ETA I always notice it because there's a terribly sad Edna O'Brien short story set at around the same period in which a young girl thinks she's actually invited to a party at a rather grander house in the neighbourhood, and gets all dressed up in a dress that was sent in a parcel by a relative who lives in the US, only to discover when she gets there, bringing a present of fresh cream, that she's essentially the unpaid hired help, carrying around refreshments and helping in the kitchen.