We had a famous company (or rather their sub-contractors) fit our kitchen
back in November.

Over the past few weeks, we have noticed a "blocked drain" smell in our
hall and I duly rodded our drains and cleared the gullies by hand.

To make sure everything was flowing okay, I filled the bathroom and kitchen
sinks with water and drained them. To my amazement, when emptying the
kitchen sink, no water flowed through the drain.

I lifted the inspection hatch and repeated the process, thinking that
perhaps they had somehow re-routed the pipes. No water came past.

I suspected then that the kitchen sink was not, in fact, connected to the
outflow pipe. It could have been like this since November, or since last
month as when emptying the sink I have never felt a need to run outside to
check it was emptying into the drain!

After some cajoling, the company sent a plumber over. He found that this
was indeed the case, and that all the water from the kitchen sink has been
going straight down. We have no basement or cellar.

Apparently (I was not in, my partner was) it had been connected, but due to
some poor welding in the original copper pipes along with a blockage, the
plumbing from the sink has become separated.

I have a couple of questions if that's okay:

1. Is that explanation likely to be accurate?
2. Is it likely that any long-term damage could have been caused? Should I
get a professional opinion?

I dread to think how many thousands of gallons of water have done down the
kitchen sink since November, especially as now we have a young baby we are
constantly washing our hands, bottles, little feeding dishes and so on.

Many thanks in advance for any thoughts.