I plan to turn one of them off each night for the next three nights

Seems like somewhat time consuming approach. You should be able to roughly judge the instantaneous consuption by the meter - e.g. by the speed of the disc if it's an old electromechanical one, or the speed of LED flash on a digital one or even a reading from the in-house display if it's a smart meter. You can then turn off the MCBs one at a time and look for a pretty immediate decrease - so tracking down the culprit circuit in a few minutes (rather than days).

   - Andy.

Or even record the consumption over an hour.

Do you actually have any three phase loads?

In your shoes with the money running out at ~ 80p an hour, £15 per day,  I'd be moving faster maybe borrowing or even buying a clamp meter to go over the meter tail and watching it while flicking stuff off, and doing it  last week to get an answer over a much shorter timescale than a whole night - consider that  you could eat out for free  at the end of the week on the money saved by faster action !!

Have you got a map of which breaker/fuse controls what loads - if not while you are isolating sections is a good time to work it out.

Mike.

There's three phases, one feeding the house, one for the garage/studio and one not in use (kept free to elec car one day). 

I know the wiring layout and have drawings, I'd really be happy to pay someone to work this through as I'm not confident in trying to do this myself. Does anyone know someone who can offer this service ? I'm near Princes Risborough in Bucks ... 

Adam, you are along the right lines already - "divide and conquer". You say that your consumption ceases when you turn off all three RCDs. Where are they please?

If you turn on each one separately, it ought to be reasonably obvious where the leccy is going. An hour's observation should be plenty to register 4 kW of power.

Then, having found the responsible RCD, you turn off all the MCBs and repeat the process with one on at a time. Start with the highest value.

As ever, photos always help us to understand the installation.

OK you have no 3 phase loads, just two groups of singe phase loads.  So if it is one RCD per phase then one of your 3RCDs should be off all the time as it feeds nothing at all. The other two should plunge one building into darkness each.

How awkward it is to turn everything off ? If you can that is fastest.  then have someone standing beside  the meter calling the readings every few minutes as you walk round putting things back on one at a time 5 mins apart?  You need to allows some time, as it is possible it is a very heavy load that is intermittent Even so, ten circuits in an hour is not too foolish.

I presume in the studio and in the main house each has some sort of main fuseboard - is it the sort with real hot-wire fuses you need to pull , or the sort with circuit breakers you can flick ?.

Post images if you like and we can chat you through it.

Again start by turning it  all off, and

First division, is the heavy load coming from the house or the studio.

Then isolate all the loads in the offending building but leave the supply to it on. Again, bring up one circuit at a time, a few mins apart, making notes of meter rate vs time as you go. 

Once you know that it is breaker 6 in board 2 or whatever that is the biggest load, then look at the drawings - what is on that breaker.? Turn it all off at the wall or unplug it, and then swicht the supply to that circuit back on. If the current still rises, there is another load you have missed.

If not go round again plugging stuff back in one at a time and see when the readings start to rise.

Mike.

Last time I heard of one of these unknown constant significant power usage problems the culprit turned out to be a modern septic tank/klargester unit gone berserk. Being out of sight /out of mind in the garden at the bottom it had been totally forgotten about that it even existed.

Thanks for the guidance !! Two updates

I turned the MCBs off that feed a shed (freezer & Hot tub) and greenhouse (Heater) overnight and it dropped by 1/3 ! (36,000 KWH pa to 24,000 KWH pa) 

I then turned everything off EXCEPT the house fridge, aga, boiler and dishwasher and it drop again to 16,000 KWH pa - but that ONLY running four items !???!?! 

I now have an electrician with a clamp meter coming out on Friday to do the testing you've all suggested. I'll keep you posted 

Really appreciate all this advise.

Hello, Aga, Greenhouse Heater, and Hot Tub will all be big consumers especially at this time of year.  You can probably expect the greenhouse heater to be on 24/7 unless it is very high power,  Aga will depend on how well insulated and draughty your kitchen is.

Out of interest did your old meter have a spinning disc?  These are generally very reliable but if they go past their certification date (which are not dealt with at all well these days) they can start to run slow if they have jewelled bearings.