Hello,
I am going through this myself at the moment.
1) If you own a 110 with GVW reading 3050kg on the VIN plate, then no matter what springs or double springs you put on it, if pulled by plod and you are over that weight then you are liable for a fine. The plate is as it came out of the factory.
2) Some 110s and all 130s are VIN GVW rated to 3500kg, but the only actual difference is the suspension components.
3) To legally carry between 3050kg and 3500kg you need to "uprate" your vehicle and get the VIN plate and the V5 altered by DVLA. The only company I have found to offer this is SV Tech. See SvTech - Special Vehicle Technology. They will give you the type approved suspension upgrades required for them to issue a new plate. It comes in at around £300 for the relevant, approved Land Rover HD suspension.
4) Only once this is done can you legally carry more weight. Plenty of people kid themselves that a set of bigger springs will do. The vehicles are capable of carrying much more than 3050kg, but legally you are limited to what that plate says, regardless of what springs are fitted. If you are involved in a crash and were weighed, the "it's got bigger spring" argument will not stand up in court.
I have found SV Tech very helpful, and I am fitting my new suspension components soon. Once they have seen photographs to show they have been fitted, they will issue a new plate at a cost of £240+ VAT. I was going to just uptrate my springs and hope I didn't get pulled, but after speaking to a copper about it he put me in the picture. Not worth the risk!
I hope this helps, I did loads of research into this so I am confident that I've got my facts right.
Give me a shout if you need any more info,
Cheers,
Adam.