(litigation) I need help compiling a defence letter rejecting that I owe nearly £20,000 for work apparently done.

In 2011 I approached a local college near where I live asking if they would be prepared to do some 3-D CAD drawings for a project I was working on. The College lecturer agreed to do the work and signed a nondisclosure agreement with his name and his being part of the College. It was the College lecturers preferred method of payment to accept royalties and rejected any fixed amount of money to be paid to him or the College. I made it perfectly clear should this product not go to market there would be no royalties to pay and he was agreeable to this.
The College lecturer worked part-time at college and was very busy doing renovations to his house. Approximately a year later the College lecturer asked if I could start putting something together in the form of a contract making reference to the amount royalties to be paid to him directly and not the College. I was concerned about this as this wasn't what we originally agreed upon. Nevertheless I went away and tried to research how much it would have cost for this project had it been done on a fixed fee basis. I've been quoted fees from £400-£1800. I sent an e-mail to the CAD designer offering him royalties up to the value of £2500. I made it perfectly clear that I did not know what the percentage I would pay him per sale would be as we still did not know the manufacturing cost, but I reiterated again any amount in royalties would never exceed £2500.
A week later and I hadn't heard anything from the CAD designer so I called him on his mobile. He wasn't very happy to say the least. Apparently he had wanted to make at least £200,000 on this project. I could have fallen on the floor with shock. To cut a long story short communication between ourselves broke down and I received an e-mail from him demanding that I pay £19,250. I ignored this e-mail then received an invoice also demanding the same amount of money through the post. I ignored this letter and then received a further invoice demanding that I pay an extra £7000 on top of the 19,250, this was an administration fee for sending out the first invoice with which I had ignored. I made it perfectly clear that I wasn't going to be paying any such amount of money as to my knowledge I was dealing with the College and not him personally. It was also patently clear that we were working on a royalties agreement should this product go to market, there was never no fixed amount of money agreed-upon to be paid for this project.
I have now received a letter from his solicitor demanding the total £19,750 or make an adequate offer to him otherwise legal proceedings will be issued against me.
I have not responded to this letter yet and would like help from a litigation specialist to prepare my defence letter. I have uploaded a file to my server which I can provide you a link to download all my files and examine my case. I believe something called quantum merit may come into play here. I have received three quotes from CAD designers for the work that is claimed to have been done by the College lecturer. The work is incomplete and is not suitable to pass to a manufacturing company; prototypes cannot be made from the drawings I have been given thus far so it is arguable do I pay any amount of money to him. Another CAD designer said that he would have charge me £450 for these drawings to have been produced and a third CAD designer said that it would have cost approximately £1800.
Basically the College lecturer is claiming it’s taken over 350 hours to produce my drawings at a rate of £55 an hour. To give you an idea of how ludicrous this is all three CAD designers that I spoke with have stated it would have taken no more than 100 hours to produce my drawings and that £55 an hour is probably over exaggerated. Remembering that this is a College lecturer using College resources, and in most part doing this work in college time it is safe to assume £55 an hour is completely inaccurate. CAD designers that pay for legitimate 3-D CAD software, rent premises, have staff and other overheads to take into consideration would charge approximately £20-£30 an hour for this type of work.
Another issue is there have never been any drawings produced that I can do anything with. They are both basically sketches of what my product could look like. In no way could I use these drawings to get my product manufactured. So if I was to pay any fixed amount of money it would be in essence dead money, I would be paying for something that I cannot use.
In an ideal world we would have a contract drawn up before any of this took place but we didn't. I can provide evidence that we were both agreeable to royalties as being the payment method.
I would appreciate any help that anybody could offer with regards to my situation. I must add that I have sought legal advice from a solicitor but I cannot afford the fees he asks, and do not qualify for legal assistance. Thank you for taking the time to read this.
Many thanks
Christopher