If you are located in the UK, a legal route could be viable even without hiring a lawyer, the case is straightforward and it is viable to present it even being a litigant in person. Even if you could afford only one trip to London and about 300-500£ in court fees it could be viable, but I am not saying it is easy to do it on your own.
Getting proper legal representation could be rather cost prohibitive, but still could be viable. Moreover, given the amount at stake i.e. > 5000£ it is likely to be allocated to fast track and therefore you could run a risk of having to pay legal costs, that could be significant, if you lose. On another hand, if you win, you likely would get your reasonable legal costs covered and likely would be able to collect the judgement (as long as Intersango is still solvent and has property in UK such as UK based bank accounts).
If Intersango tells you something like: "give us such and such docs and problem solved", then basically your best bet is to give then what they want. Whether you want to deal with them after that is up to you.
If Intersango tells you something like: "we will keep your money because we suspect you are a fraudster", then "put up or shut up" is basically the only choice you have. You either take it lying down or sue them.
Collection agencies in England are useless, toothless tigers, not worth even trying. This is exactly because they will just send 1000 letters (at your expense) but generally will not go to the court for you and will not do anything else actually.
I doubt that police would help you at all because they probably will say it is a civil and not a criminal matter. FSA (
http://www.fsa.gov.uk/) and The Financial Ombudsman (
http://www.financial-ombudsman.org.uk/) are two other authorities that are in position to help you (or should be).
FSA will tell you that they only deal with companies that they regulate and the only thing you could tell them is that they shall ensure that they regulate Intersango (if you think they should). I do not know if they would care or not and whether it would help you at all.
Both with FSA and The Financial Ombudsman route the procedure is simple. You ask intersango for their formal complaint policy, lodge a complain with them as per such policy (if any), if your complain is not resolved to your satisfaction within 2-4 weeks you can lodge it with FSA and/or with the ombudsman.
I'd say
https://www.moneyclaim.gov.uk/ is a better point to start. One thing that you have well protected in UK is your fundamental human rights, including the right to justice. If it was my account treated like this (as far as I know the circumstances from OP), I would sue them in a heartbit.
P.S. I am not qualified to give legal advise (and do not expect to be qualified for a few more years). Nothing in my posts should be treated as legal advise.
P.P.S Once there is a bitcoin exchange based in uk and regulated by FSA it makes sense to start using such an exchange because both The Ombudsman and FSA will be extremely effective in helping you enforcing your legal rights should there be any conflict with such company.
