As its 'foul water', you can't just let it run off into the garden. You would have to let it overflow into a septic tank, which you would then have to subsequently have to have emptied (for a fee of course!). Is the drain cover inside your conservatory or outside? If its inside, you can get screw down covers which are more or less airtight (quite often used in public toilets etc). If its outside, I can't see how it floods the floor, as the cover should be at least 6" below the conservatory floor. This could be classed as an environmental health issue, in which case it might be worth getting the council involved. It would be possible to put an 'overflow' pipe at the top of the chamber if its outside, but if its inside it would mean digging up your floor. Tricky to give any good advice when you can't see the situation 'in the flesh'.