No terms and conditions and Letting Agent changed name owing money?

About 3 years ago as a total novice in the letting business I requested that an agent manage my property.
The Letting agent did not give me a brochure when I met him at my property nor did he send me one. All my communications with the Letting Agent were subsequently done by email.

I signed a one page agreement which failed to mention the most basic information which at the time I thought was some kind of preliminary agreement, to which a proper contract would follow. No further document was issued and I got on with my life, happily receiving rent every month.

It is not until the Letting Agent increased its management fees by 50% and referred to terms in some brochure that I realised that I have no brochure to go by!

I also realised the Letting agency had changed its name and the contact person has left the agency. The Letting Agent failed to pass on 3 months of rent and wants me to pay the equivalent of 3 months rent to come out of contract, something which I had no knowledge about and is not mentioned in the one page agreement I signed.

I terminated business with the company and opened a complaint with the Property Ombusman. What I would like to know is: since the company I signed the so-called agreement with is different from the current company, what are the legal implications?

Other than the Property Ombudsman where can I complain about this company? (I want to leave court as a last resort)?

I would be very grateful if members on this forum could give me their opinion and advice on the matter.

Thanks and regards,

Louisa