James the problem would be if you just returned another 16mm from the last load point back to the DB, though you have created a Ring Main you have also created an unbalanced ring main, as there would be very little load on the return leg, and you will still have the same problem, perhaps an even greater problems as you would be increasing the protection device size, thinking that your having a balanced Ring, and the increase of load on that 16mm will give rise to temperaure on the outgoing leg cable and could result in damage.
Unfortunately because your wanting to keep a single protection device for the 3 plots you can not run a parallel cable to supplement the existing cable as you will fall foul of regulation 433.4 regarding isolation or switching within the circuit.
There are ways you can do this, create a ring main and balance out the load, but with just 3 loads that will make it difficult but not impossible, Kerchoffs law is your guide there.
If possible can you utilise a 3 phase supply ?, that way you can run a new 4 or 5 core SWA which could be smaller than the 16mm and give you a better balance.
Then of course increase the size of the 16mm to say a 25mm .