You can never ever be 100% certain its safe to do so in that kind of situation, the court will see that the OP did everything possible/correctly according to the HWC and that if it wasn't for a nut job biker violating multiple highway code rules the would have been no accident, I mean look at all this:
The DO NOT is a legal requirements and violation constitutes a criminal offence (hence my illegal overtake comment) the DO NOTs may be used in evidence in any court proceedings under the Traffic Acts (see page 124) to establish liability.
Assuming the witness will go to court all the OP needs is semi competent legal representation and some testicles and he will walk it.
Proving the biker was partially at fault does not mean the OP wasn't driving dangerously.
This isn't an insurance claim, it has 0 to do with the biker now. It isn't about pointing blame at 1 party, or 50/50. The police are interested in weather the OP was driving dangerously or not, he pulled out when he could not see clearly the road he was pulling out onto.
What he should have done was told the lorry to continue driving and only pull out once it was painfully clear both sides of the road were totally clear.
In reality this never happens because everyone would spend 10 minutes at every junction, but a court doesn't take into account common sense. It deals only with the law, and I would be very surprised if the law found the OP was driving totally safely - when he pulled out onto a road he could not see and was relying on a lorry flashing him to say it's okay to pull out.