You could have a look at this device.

https://www.pqwtdetector.com/PQWT-L7000-Multi-Sensor-Water-Pipeline-Leak-detector-pd44173515.html

I would contact your home insurance company they would normally cover this with their track and track service I would have thought.  With 400 ltrs a day thats got to be a big hole.  

You could call a leak specialist out yourself if you don't want to go through insurance and some offer a no find no fee service. 

@soniks, the leak size is a guesstimate on what I’ve “measured” our consumption be versus what the metre is saying. It’s crazy we can’t see evidence of this, and I’ve mapped out the pipe work in my head, and can’t really figure out where it is. We were out all day on Wednesday and there was nothing running with water - we ‘used’ 300 litres while we were out.

The home insurance is a good idea, but not sure if they’ll accept the claim based on a meter reading and no evidence.

@editor would the 'evidence' not be the water bill given there are two persons living in a domestic property and you aren't watering your extensive farm land at this time of year.  The swimming pool isn't leaking is it? 😀

@editor Hi

I'm sure you probably checked everywhere and tried some things. 280ml ish of water pe minute is not that much in order to be visible, there are two types of soil, one that lets water drain easily and the other doesn't. Hence no visible signs.

Given the small amount, have you tried turning off the water supply to everything connected directly to the drainage system? toilets and machinery mainly.

Could it be a faulty meter?

Posted by: @fazel

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@editor Hi

I'm sure you probably checked everywhere and tried some things. 280ml ish of water pe minute is not that much in order to be visible, there are two types of soil, one that lets water drain easily and the other doesn't. Hence no visible signs.

An interesting way to look at it. I’m going to seal off some taps tomorrow and see where that takes us.

@soniks, yes, there’s a possibility the meter is broken. Not sure how they’d test it, but worth raising I can’t find a leak source.

@editor

You could probably check the accuracy of your meter by taking a reading, and then filling a vessel of know volume, the larger the better, in a short period of time, then obtaining a new reading. Subtract the first reading from the second one and see how this compares with the volume of the vessel.

Try not to use any water, or try to measure the quantity of any used, whilst taking a meter reading at hourly intervals, for as long as possible. If you suspect that you are loosing 300 litres per day, then this would equate to approximately 12.5 litres per hour. If your meter reading is increasing by approximately this amount each hour, that would tend to indicate a continuous leak somewhere.

@derek-m, good plan. I’ll do that today and use the bath as the large test vessel tonight.

Data collection has been semi successful today. One weird thing is that there isn’t a consistent water loss… at times it worse than others. But there’s definitely a leak. 1000%. I’m going to collect data for a few more days and post to see if anyone can identify something that may hint to an issue. 

Posted by: @editor

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One weird thing is that there isn’t a consistent water loss

Perhaps the leak after a float valve and flow therefore intermittent?  Do you have an vented hot water system or some other system with level controlled via a float valve?

@ronin92, we have a vented hot water cylinder, but no clue if there are any level controlling float valves.

Just to give you an idea, we watched TV tonight with no water usage (washing machine, dishwasher, etc.). First 40 minute segment, we registered 11 litres used. Went back to watch TV. Next 40 minute chunk was 48 litres - more than one litre per minute. Scary. I’ll take a reading tonight when we go to bed and see what it registers in the morning.

Posted by: @editor

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@ronin92, we have a vented hot water cylinder, but no clue if there are any level controlling float valves.

Just to give you an idea, we watched TV tonight with no water usage (washing machine, dishwasher, etc.). First 40 minute segment, we registered 11 litres used. Went back to watch TV. Next 40 minute chunk was 48 litres - more than one litre per minute. Scary. I’ll take a reading tonight when we go to bed and see what it registers in the morning.

If you have a vented hot water cylinder, then you will probably have a supply tank in the loft, which will have a ball cock valve supplied from your water main. There will also be an overflow pipe connected to the tank in case the ball cock valve does not fully seat.