My concern is you say it was for a caravan, with caravan sites your not allowed a TN-C-S supply, so it is possible that the supply is TT to be used with the caravan, with caravan sites fire regulations mean the caravan is some distance from building, but at home often they are closer, so it is down to the electrician to decide if a TT or TN-C-S supply is used. This is selected in relation to things near it, with where my caravan is parked at my father-in-laws house the meter cupboards mean the two earth systems would be within arms length of each other so the TN-C-S system was used to the caravan.
By moving the supply you may be causing problems with the earth system, it does seem a bit odd to put twin and earth in conduit, outside the norm would be SWA, that is not saying twin and earth in conduit is wrong, the system has to be designed not simply thrown together. I had to get the electricians back to mothers house because of a twin and earth supply to the garage, now changed, electricians are like anyone else, if they have twin and earth on the van and to get correct cable they need to go back to yard there is a temptation to use wrong cable, in conduit it should not be a problem however. But if metal conduit then the earth on the conduit will need testing as well as the earth in the cable.
Since a caravan is supplied with a 25 meter cable of 2.5 mm one would expect that you could extend by 25 meters if the earth wire is 2.5 mm but with twin and earth the earth wire is only 1.5 mm.
So in conclusion it is unknown if extending the supply will cause problems or not, one would need the results of an inspection and testing, which you may have, one should have inspected and tested work as far back as I can remember, but it was only when BS7671 came out (1992) that electricians started following the regulations to the letter, for the first time electricians were tested on their knowledge of the book, before then is was unusual to see installation or minor works certificates raised, with Part P there was a further push to fill in forms and actually issue them, (2004) today one would hope no electrician would do work without inspecting and testing and issuing forms.
To be fair in the days of fuses if a circuit was a little too long then it just took a little longer to rupture the fuse with a fault, but when we moved from the humble fuse to the MCB we had something very different, the MCB is two devices in one, there is a thermal overload and a magnetic overload, for the latter to work the loop impedance needs to be below a set figure, so unlike the fuse, there is a step between tripping in 0.01 of a second and 5 seconds if the loop impedance is exceeded. So today there is a real need to test. There are some cheap testers like the EZ150 from Martindale at around £50 but these have just simple lights, so will tell you after finished if within limits or not, but will not tell you before you start if it is likely to be within the limits.
As to informing LABC before you start and paying fees, the fees when I did it were £100 plus vat plus any inspection and testing fees for first £2000 worth of work, that was 2006 likely more now, for such a simple job it would not be worth it, you would likely get the whole job done and self certified by a scheme member electrician for less than the LABC fees.
I am sure many people do work without notifying and without inspection and testing, but because you say it was a caravan supply, in your case I would be wary of doing DIY.
