Housing - my deposit was lost by letting agency

introduction
Ok - I will try not to make this too long winded, for the sake of keeping it anoymous I will refer to Landlord and Agency rather than thier names - landlord is the owner of the property in question, agency is a Letting agency taken on by the landlord.

Situation.

Last year (may 09) i moved into a room.
Landlord was to manage tenancy but had a tenant finder - Agency . Agency were to check refs and handle deposit.
I informed both landlord and agency up front that i would not be able to provide a deposit cheque right away and we agreed a 3 month time to get it - i got a donation from a charity fund (the reasons why a charity would pay out are irrelevant and personal)
Also agreed was that i would be one month in arrears with my rent due to using housing benefit which pays one month in arrears.

Charity fund sent me a cheque in very early july, signed 30th june.

Landlord came around (he came every 2 days and knocked on my door until that point asking for the deposit and the month rent i was in arrears even though we had agreement on it.)
I told Landlord on morning of recieveing cheque that it had arrived, i opened cheque - it was written out to agency/landlord i thought it might be hard to cash because it had both names on - landlord said he had to go to visit agency on another matter and took cheque to talk to them, i have a witness who saw me hand over cheque.

I heard nothing more - was foolish and assumed it was all ok.

tenancy ended - i was still one month in arrears with my rent - i admit to this and agreed to write over deposit to landlord. at this point landlord tried to say i owed him nearly a thousand pounds - this was not true but this was resolved, i paid the difference between rent arrear and deposit cheque and wrote to the agency to tell them i was allowing landlord to take deposit for last bit of rent owed.

I moved out - all was well

1 week passed then landlord rang me - said no deposit ever paid. i reminded him that i gave him the cheque also rather curtly reminded him that there was a witness in room when handed it to him because i was immediately alert he might be trying to make a scam.

Landlord rang back - explained that agency have to by law open a seperate account for my deposit - it was empty. I remind him again he had cheque last - tell him he must talk to agency, he agrees.

I hear no more until yesterday (6 months passed)

Yesterday my garountor for my rent got a letter saying i owed one months rent and that he had to pay it or they take him to court.

I phone landlord - he accuses me of changing my number (its been same number for 5 years!) says he sent emails - i have none! says he told me to speak to charity to confirm cheque - he had not (although i realise now i should have done this anyway - but i wasnt expecting this) I confess i lose temper and shout >.< not productive i know >.<

I phoned charity trust who wrote my cheque - asked them to run checks to see if it was cashed.

This morning charity trust ring - cheque WAS cashed!! - also cashed in august! a month and a half after i gave it to landlord to take to agency! they agree to send me this in writing as proof.

I ring landlord - he says all is my own fault! (i dont know how - but i did yell at him yesterday so maybe just still angry - i did apologise for shouting) he says only agency could have cashed it - he says he will talk to them but i am unhappy so...

I ring agency - demand an explanation. agency woman very nice about it - says she will check her bank records to see if it is down as being cashed by them or not but tells me that landlord COULD possibly have cashed it too.

my questions

1, who could legally cash this check written out to agency/landlord - both? or as landlord tells me just agency?

2, if agency say did not cash it what is my next step? legal advise? can they/ do they have to PROVE they didnt cash it?

3, what if agency do find they cashed it - it just ends? they get away with this huge breach of confidence?

4, can any money be legally taken from my garountor whilst this mess is being sorted - i do owe one months rent - but whoever cashed this cheque owe me a deposit (same amount) - its very confusing.