I rang Stainless Exhaust Specialist on 23rd Sept and spoke to Steve. He was very friendly and told me that I needed to send them a letter containing the order and with a £50 deposit by cheque. He further said that they will contact me to send the balance once they have manufactured the exhaust and shipped it. In my case it will go to PJ Grady for Chris N to fit.
I do recommend calling and asking to speak to Steve first (he actually was the person who answered the call when I rang). Alternatively you could just send your order via a letter with the deposit.