We need to renegotiate if we're to stay in the EU

Ken Clarke's recent comments on The Politics Show have raised once again the issue of what should be done if the Lisbon Treaty is already ratified once we enter office.  A recent ToryDiary suggested that Hague has been exhausted by negotiations for a new MEP group post-EPP, and regards the European issue as a ticking timebomb at the heart of the party, best not messed about with lest it go off.
 
My experience of Conservative party views on the EU is roughly as follows: 40% wish to leave the EU; 50% wish to renegotiate our position within the EU, giving no more power to Brussels and taking back some of that already gone; 10% are content with things as they are and as they are headed.  If I'm right, then there is an overwhelming majority within the party favouring significant change.  "Leaving things much as they are" is precisely what would set off the timebomb.  To not propose significant change would be to side with a tiny minority.  Opposition to the leadership on the issue would very quickly come out into the open, potentially derailing Cameron's premiership.
 
More than that, at the national level it appears in opinion polls that there is a growing appetite to leave the EU.  I suspect that the Cameron administration is the last chance for the UK's long-term membership of the EU.  If Cameron can successfully renegotiate our position within the EU, then there is a chance that the UK will still be a member in thirty years time.  If Cameron chooses not to try to renegotiate, or if he does try but is not taken seriously by our EU partners, then I expect that the UK will have left the EU within ten to fifteen years.
 
So the options for Cameron are: renegotiate and try to stay in; or don't renegotiate, create massive splits in your own party, destroy what little belief there is among the British public that any of our politicians will ever grasp how discontent voters are with the current trend in the EU, and wait for the inevitable overwhelming momentum for withdrawal that will follow.