I would think having the £100 per day late payment charge would cause more problems than it would solve.
1. Makes you look like a loan shark
2. Would get your business a reputation
3. would scare potential customers
4. More likely to get customers falsly accusing of poor work to avoid cost because they forgot to pay on time.
5. If they accept the terms they may well do so because they have no intention of paying anything anyway!
6. probably un enforcable.
7. If you were ever to charge the £100's you would lose them as a customer.
I fully understand why you have done it and sympathise with the problem of late payments, after all its your money!!!!
Other people on here use a percentage discount for prompt payment and i think it may work for you also.
As for speaking to the director it sounds like you mean via telephone? either way I would at this stage get down to the office and if he cant see me there and then, "no problem, i will wait!". The account needs settling and firm direct action to asses whether or not they intend to pay would be the first thing i would do. There is no real need to threaten with credit collection companies as i dont think it's a threat taken seriously by people (everyone makes this threat). I would just explain what your going to do (small claims court), send final demand to the letting agency stating court action will be pursued after 7 days of non payment (keeping a copy). Then if they dont settle within 7 days just file the court form and forget about it, once they recieve the court claim they will pay it (usually!!).
Remember to be firm and direct, dont get into a debate (nothing to debate, they owe the money), put any communication in writing keeping copies and just get on with it. Once they see you a clear minded, determined and business like, you put your self in a good position to succeed.
best of luck
steve