With regard to the forged documents, this is clearly attempted fraud although you may find that the police take the line of least resistance and say that this is a civil matter between you and the designer. However the fact remains that it is fraud. The police however may take more notice because of the previous conviction although because it was suspended for two years, it will only be taken into account with regard to the sentence that he receives if he is convicted again but will not be added to it. It will make any penalty more severe.
Under the Supply of Goods and Services Act, if no price has been agreed before the job starts, then the price must be reasonable. In order to ascertain what is reasonable I suggest that you get something from another designer in the form of a letter or a quotation explaining exactly how much the work that is being done should cost.
Armed with that, if he will then not refund however much you think he has overcharged, you are going to have to issue court proceedings against him and that a judge decide the matter. Whether you win or not will depend on how much evidence you have which confirms that he has been overpaid.
If he denies getting cash payments, if you have bank statements which show the money coming out of the bank on a regular basis, that will really help you.
Can I clarify anything for you?