It was 34% of those eligible to vote in the last referendum. I think it goes down considerably from that in any future referendum.
Of course just my opinion!
Its an interesting point... I think the vast majority of those that voted remain before would do so again, the vast majority who voted leave would do so again... some small shifts and of course demographically more of an advantage to remain as probably more young (generally remain) voters and fewer old (generally leave) voters... the three real drivers of success would be
1. Who can vote (eg lowering age to 16 like in indy ref, or allowing eu citizens resident in the uk to vote)
2. what the question was (it probably wouldnt be the same as it didnt resolve anything before)
3. that approx 33% of the population that didnt vote - there will be remain and leave people in that and energizing those would seem to be the best way for either side to get the vote up.
I have to say an appeal to base emotions probably is better at energising people than pragmatic arguments of economics
But in all of that the real thing to consider is will a GE or a referendum come first ... if its a GE then forget about all of the above because with a brexit/conservative alliance being the leave option, the remain vote split between several parties and a FPTP system there is a very good chance that there is a WTO commitment in a manifesto and a big enough majority in parliament to force it through
basically unless corbyn changes his mind on 2 things (backing somebody else to lead a GNU and a second referendum before a general election) I think leave on WTO rules is odds on (with as you say only a small % of the population actually voting for it)
I am positive that in the long term the libs will probably become the european democrats and eventually (10 years or so) we might get to rejoin having had a decade of lost growth and having to accept the Euro, no veto and a European Army - but in truth im ok with all those things and if my son gets freedom of movement back and European Citizenship around his 18th Birthday then im more than ok with that - Id rather win the war than win the battle and after the smug we got our country back and hoopla a decade of econnomic difficulties, a growing demographic in favour of being in Europe and not to mention us remoaners continuing to remoan with a good dose of told you so with each economic problem Im confident we ultimatley end up back where I think we belong and back in Europe ... but as for this particular battle - unless there is a referendum before a GE i think its probably going to be lost