I've recently bought a Lrp 2s lipo for my sanwa tx. When I connect it to my balance charger the total voltage goes straight up to 8.4 from 7.6 and stops charging as it is full, however each cell is still reading 3.8v not 4.2.

It sounds like you have some high resistance somewhere in the main lead circuit. It can be connections, skinny wires, small connectors, ect.