Thanks for replies so far:
@butlers - sump pump yes 😉 and no, there are down pipes or guttering above that wall (side/end of terrace)...
@silverglade5 - we thought about water table so waited and monitored it for a couple of weeks, but hasn't really rained past 2 weeks and still coming through as quick if not quicker than before...
@dan2802 - yes contacted YW today, chap came out, said is clean water (not waste), no immediate explanation where its coming from & has said will add us to list for 'clean water team' (?) to check up the street in the coming days...