Hi Bell, sounds like you could have a case since you paid the money and they failed to pass it onto the correct people within 14 days. They owed lots of people money (LL, tenants, tradesmen etc) and they will not have all just accepted that GDH have 'no access' to the account. If you have kept your receipt that will really help prove your point.
The only problem you may have, not paying your last month and a halfs rent, is that Jupiter take about 10% of the rent you pay as their 'fees' before passing the remainder on to the LL, so for example if you pay £495 rent, Jupiter take about £50 and the landlord gets the other £445. So, jupiter may get angry at not getting their money, however after they took your money, knowing the situation the company was in and have not tried to help you get it back, the least they could do is let this £50 go.
I would definately attempt to get the money back tho, we have been waiting for 6 weeks now for a decision to be made by hte TDS but it will be worth it if we get the money. If we didnt, I would certainly take legal advice and attempt to take them to court, but this is mainly because our LL has not helped us at all, least yours is cooperating!
Maybe he could put something in writing, confirming what you have discussed, so that you are happy you have a guarantee, and he cannot change his mind at a later date. Good luck!