After investigation I've found the soakaway/French drain that my roof guttering exits into has become ineffective - the water from the down pipe doesn't actually drain away - indeed in the recent heavy rains it backed up, in the down pipe, to the roof level! The situation is made worse as my down pipe is the only one for the four houses in the my terrace.
My question is are you allowed to route the rainwater from roof gutters into the normal household drainage system (where sink, bath and toilet waste water goes)?
The external man hole cover is just 4 foot from the down pipe. It would be a much simpler job to route the roof gutter pipe work into the standard drains than create a big enough soakaway to coupe with the amount of water from the 4 roofs.