I was waiting at a junction to turn left onto a main road and as the car coming towards me from my right was indicating left and looked like it was slowing down I assumed she was turning into the road I was exiting. As I pulled out she knocked me off my bike. Her insurers are claiming that it is 50% my fault which cannot be right, can it?
There is quite a lot of case law on this topic but the case of Winter v Cotton is closest to the facts of your accident as that involved a slowing down vehicle that was indicating, like in your case. Winter relied on both the signal and Cotton's slow speed and the court decided that Cotton was 100% responsible.
The Highway Code makes it clear that a driver on a main road has right of way but the court will look at all the circumstances surrounding the accident including signalling, behaviour and witness evidence. The advice to those emerging from minor roads is to wait and only do so when you are 100% sure it is safe to pull out.
You should press for 100% of your compensation based on Winter v Cotton but do be prepared to accept a reasonable offer bearing in mind the ever present litigation risk, as other cases like this have not ended up 100% in favour of the driver pulling out.