Question about drainage - probably for a builder!

[mogwai]

We live in a 1931 semi with suspended timber floors in the living room/dining room. We have lived here for 11 years with no previous problems of this kind.

Last month we detected a foul smell coming from under the floorboards in these rooms (it smelt like sewage). There are no pipes under the floor except for the central heating radiator pipes but the central heating has not been a problem and we have lost no pressure.

We removed some floorboards and found a sort of drainage gully moulded into the concrete foundations of the house. We were previously unaware of this gully and don't know it's exact purpose or where it is draining to/from.

We hired a water vac and removed the water (about 30 litres removed). This has left a white slimy residue. The smell is improved (you can't smell it when the ground outside is frozen) but when the outside temperature rises the smell returns. It's not terrible but bad enough to warrant spraying air freshner.

We are unsure what to do next. It has been ten days since we removed the water. Do we wait a while longer to see whether the smell subsides? This doesn't solve the mystery of why the water was in the gully or where it came from.

The possible explanations include two very wet summers (we are on a clay soil and our back garden becomes waterlogged very easily) or a new patio which was fitted last march. This is the second patio we have fitted in eleven years. The previous one had a gap around the house filled with slate chippings. The new patio abutts the house but remains below the level of the damp proof course (though about an inch below the air brick). The patio has been laid on a slope to direct water away from the house.

Any advice gratefully received. What sort of professional would we approach to investigate formally?

Thanks

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Dh Used to work in drainage and says it's very unusual to have a gully running under the house. He suggests getting hold of a drainage company and getting them to do cctv rodding in the gully to find out where it goes/comes from.
HTH

oh that's a good idea. It must go somewhere....

My DP is a builder and he suggests the same as sirius - you need a cctv check.

He did say that white, smelly residue is usually fat - which suggests drains. Could it be that there was an old drain - from a now hidden scullery perhaps?

no, the house was built in 1931 and existed in it's original form until 2004 when we added an extension. As we added the extension I saw the floor excavated in the old kitchen and know there's nothing under the house.

The entire house is concrete-based with the exception of these floorboards in the living room/dining room.

It's a total pain in the neck. Our floorboards were french polished a few years ago (cost a fortune) and every time DH lifts one or two of them he damages them. Feel sic at the thought of some drainage firm coming in (or is that just the smell.....)