I really feel for you, now as usual SJ has coverd everything in great detail, but if i can add a few things
re the benefits, as has been said, be honest, at the end of the day thier main interest is getting the money back, and as you say its only a few months its not a great amount, so its unlikley they will make a big fuss of it
now the tax, i know next to nothing, but found this link which may help
http://www.insolvencyhelpline.co.uk/tax_debt/index.htm
as your living with your parents, its a big step, but i would actually look into the bankrupcy option, but do get professional advice, there are so many rules and situations, i couldnt possibly list them here, but you can be discharged in as little as 12 months, depending on individual circumstances etc etc, and it doesnt now carry the same stigma it once would
I know it looks bad at the moment, but things really arnet that bad, give it time and it will all be a bad memory