where do i put this?

had service at local ford dealer yesterday with my 56 plate ford focus 2.0l tdi. i have an engine system fault light on. he told me that it was fluid that is sent into the cat converter to clean it out. he said it would cost £75 per litre for the fliud and i need 2 litres then £100 for them to do it as its highly corosive.
can get liquid from local car parts dealer for £8 per litre but where does it go??? help dont want to spend £245...

mick beevers
May 2011