The Census district might well be Onchan but Bigwell street was in Douglas (now approximately Lord Street) - St Barnabas's, now demolished, was on the small carpark to the town side of what was the bus-station and about 100 yards from the Sea-terminal; the ropewalk was probably the one where the railway station/Tesco's supermarket now is - easy walking distance (400 yards or less) down from Bigwell street.
William + wife + family were all born in Douglas according to their entry in 81 census - St B. was built 1835'ish so previous generations were, if living in Douglas, probably baptised at St Matthew's