Several things could be causing this Iainw, As its new I doubt its an underground spring? But have large roots damage a drain? or is it run off from a new drive/patio? Or is there a blocked /leaking down gutter or drain? You need to have a good look and see where it is coming from if you can?

I agree, DRC, - I found an old sink buried in mine a few years back. It explained a lot!

Made a good reservoir OJ just where you dont need it! must have taken some digging out?