1 Is there a legally responsible person in a caving group/party even if there is no designated leader ?
2 If so, what parameters define that person ?
3 If a 3rd party then provides information/instructions which a responsible person then follows, is the 3rd party accountable/liable if an incident occurs thereafter ?
4 Also, has there been any incidents where a person in a group has been held accountable where they had not assumed responsibility beforehand?
5 Lastly, is the same true of mines ?
Re 1 simple answer is no. Every one bears some degree of responsibility to every one else or as it is know "a duty of care". But this duty of care is proportional to the degree of prior expertise. So some one who is say highly experienced would be considered to have a higher level of a duty of care compared to a person who is less experienced. This is tempered by self evident knowledge like if being asked to jump down a 10 foot drop, one is clearly likely to end up being hurt, so your own common sense should have told you not to do it. (Unless of course you are a vulnerable person who may be considered to be deficient in such judgements when the onus moves back onto the more experienced person.)
Re 2 So no is the answer.
Re 3 Yes to a limited extent. That is why guide book writers need to be careful. But whilst I think you were right in your responses in the situation you outline, I also think you would have been wrong if on being asked "Is the cave flooding?" you said nothing when you knew that it was. I trust you can see the difference.
Re 4 Don't know, since don't track such legal cases. But my guess is yes.
Re 5 Whilst the above is true for mines as well, if I recall correctly, there are some statutory duties on the "mine owner" which they cannot get out of.
You may wish to follow up
http://en.wikipedia.org/wiki/Duty_of_care_in_English_law for the heavy stuff on duty of care. I will cite from it
"The rule that you are to love your neighbour becomes in law, you must not injure your neighbour; and the lawyer's question, Who is my neighbour? receives a restricted reply. You must take reasonable care to avoid acts or omissions which you can reasonably foresee would be likely to injure your neighbour. Who, then, in law is my neighbour? The answer seems to be - persons who are so closely and directly affected by my act that I ought reasonably to have them in contemplation as being so affected when I am directing my mind to the acts or omissions which are called in question."
You could also read Legal and Insurance, see
http://bcra.org.uk/pub/cs/index.html?j=12 which is somewhat lighter.
Now for the legal bit - I am not a lawyer so don't rely upon this advice. Sorry about that.
Bob