The money has gone on improvements to the house which the landlord had agreed, decoration and such.
So you spent the equivalent of 6 month's rent on improvements to the property with the landlord's permission? Do you have his permission in an email? Did he know you were spending the rent on enhancing the value of his asset? And do you have receipts for everything you spent?
I know these are a lot of questions and you are feeling a bit overwhelmed by it all, but I am shocked that this is being allowed to happen.
Also - how is the mortgage for the landlord's father's property in any way tied up in his business and connected to a business partner? Is the partner his wife?
I would go in all guns blazing about the rent money going on repairs and make him put everything in an email:
Dear Landlord,
Thank you for your email yesterday [or whatever].
Just to clarify this in writing, I was willing to pay the next six month's rent on the 15th of October, when it was falling due.
However, after our conversation on the 18th of October when you said that you were happy to switch to monthly payment instead, I used part of that money to make some upgrades to the property, which you agreed to.
These included:
Repainting XYZ - £250
New ABC - £140
Replacements for DEF - £540
[etc etc etc]
Total: £XXX
Please see receipts attached.
Naturally this means that the £XXX for the rental period November - May is no longer intact.
As you know my personal situation, you must be aware that I cannot get a guarantor. I am surprised that you waited nearly a month to tell me that you had changed your mind.
You must also be aware that I have spent part of the 6 month's rent for the next period on improvements which were done on the clear understanding that I was to remain in the property for the next three years, and that you were happy for me to do this work and that you were happy with monthly payments.
If this has changed, I would like to be fully reimbursed for the improvements immediately, or have it deducted from the next 6 month's rent payment. I can then pay you the balance for the next 6 months up front as per our original agreement.
If this has NOT changed, I would like a new lease specifying these terms that you verbally agreed to in October.
Best,
Ikeakia