Exactly what are you intending to install in these holiday chalets, and how many were you intending to erect/build?? To be totally honest, this predicament you find yourself in, is of your own making. You don't lay any cable in the ground without first knowing what that cable needs to supply and doing the calculations. So if you haven't done your homework and given the details of the cable size required to the seller, that's not the seller's problem is it??
What's more, you haven't stated whether this new supply is 2 core, 3 core, 4 core or if we're talking about single or a 3 phase supply!! No wonder you're now in a right old pickle.
We own one plot on the development site. There is 8 plots in total. 2 plots were purchased by one party and the owners of these decided to commission Scottish Power to take a 95mm cable (aluminium) from the transformer to feed these two lodges. I believe SP then feed 25mm off this 95mm cable. They have full spec electrics for 3 bedrooms, 2 bathrooms and electric heating. There lodges are around 180m2 in floor area.
We are building one lodge on our own plot that we bought. I believe there is a 11KV transformer on site which 8 plots plan to take supply from (2 have already done the above) I say lodges/chalets but it is actually a timber framed high spec house which we are building. One plot owner has decided to build a small 2 bed chalet though so the 10mm may be sufficient for them as they are half the distance from the mains transformer.
We bought the plot with the agreement the land developer laid the infrastructure and connected us to the mains water and transformer. Hes already wriggling out of the connection for electric saying it is our responsibility. If you or anyone else would like to see the clause on this then I can post this on here and I would very much appreciate your advice.
Also for your information he messed up the water supply. The way he did it meant that there was no water pressure during the day time in peak season (tourist area) and the water pipes ran dry. He forced the plot owners to buy into a ÂŁ12,000 scheme to rectify this. By using a series of break tanks, header tanks and pumps. We are doing our own individual water tank scheme in our property which will fill up fine during the day when demand isn't high or at night time. Really the seller should have rectified this as he sold plots with a water supply and there is not a sufficient water supply to them? And this is why we did our own scheme. The other plot owner who has 2 chalets as mentioned above decided to go with the scheme as they intended to rent theirs out as holiday accommodation and needed a 100% reliable supply.
The 10mm will be fed from a split phase supply from the mast (I am unsure on the size) this then would feed into a 8 way BEMCO box. Which each 10mm three core cable would connect into for each plot. In our case 100m to our plot which I now know is severely inadequate.
To also throw it out there, we bought the plots with SEPA consulted and agreed systems for soak-aways. Our plot has failed the percolation test for this (granted on day when it was record rainfall) we will also have issues with this and more expense for something we bought which was meant to have a agreed system.
We have been naive, and have never undertaken a new build before. We thought the seller had done his research as he implied and everything was adequate. We have 30 years experience in property development and construction so fortunately the rest of the build will be under our control and should go smoothly.