It seems that nowaday's insurance companies make quick agreements on a %age basis of fault - whether their client agrees with it or not. For example they don't seem to do knock for knock anymore, instead they total the cost of repairing all the damage to all parties (inc 3rd parties) then divid it by 50/50 to each party or whatever quick %age is agreed (in your case 70/30).

I know that this was clearly not your fault but the insurance companies do it because it means they never get into arguing with each other, its quicker than looking into the facts properly (hence saving legal costs etc), and on top both parties lose their NCB/or are marked 'at fault' so both end up paying higher premiums. Bastards eh .

I'm sure you could keep fighting this though, but as discussed above your insurance co will have little or no reasoning to do it for themselves - so you need to keep pushing them on this