The house is in west essex, so not much clay that I'm aware of and is around 11 years old.
The cracks in the plasterboard are only immediately below the window, they are not visible on the ground floor at all.
The company didn't point out about the lintels until very recently when I asked...
I have recently noticed a crack running from the corner of an upstairs window (new) vertically down to the top corner of our patio doors(existing). The crack runs through a couple of bricks but is 1-2mm wide at it's widest. There are no cracks above the upstairs window at all and no other cracks...