Hi Richy,
This could get very nasty!
Firstly, it is a legal requirement for you to provide a valid risk assessment, I notice that you have a Health & Safety Policy Statement on your website, part of this states that you will ensure that your own staff and any subbies will adhere to all the legal requirements.
Your client, has a duty of care to ensure the Health & Safety of its own employee's, sub contractors, memebers of the public, visitors etc. So they should have ensured that you hold valid insurance, asked for risk assessments, method statements etc. You should have asked your own sub contractor to supply the same, as you of course hold the same duty of care.
Your sub contractor should have refused to do the job, as no one has supplied any risk assessment stating that the ladder needed to be secured, either by footing or other means of securing the ladder.
Of course if any of the companies do not employ more than 5 people, you may be able to argue that under current legislation, no written Health & Safety Policy needs to be produced, only a Policy Statement,and you may be ok on this point. However, each of you who actually hold insurance, may be asked by your insurance company to provide your full written Health & Safety Policy, to help each insurer decide liability (ie. who broke which rules etc).
As stated by mike, all of you could be held jointly liable for this, it really now comes down to the woman who was the victim of this accident, if you are really unlucky, this lady could well take each of you to the cleaners, quite literally! Your insurance company, may well refuse to pay any claim made, due to the fact that no party involved in this actually excersised any duty of care at all, which would no doubt result in prosecution, and a heavy fine, as well as still having to find the compensation.
I dont think that your insurance company will swallow the fact that you didnt know that your family member was not insured, cant see them liking that at all!
As Mike said, contact your insurance company, and let them deal with it, and just hope that they will cover any costs, and give you the help that you need.
Best of luck!
Regards,
Rob