Hi all, I have my 2015 b8.5 S4 Avant (26.5k miles) at the local Audi dealers for it's oil service change and they've sent me an Audi video showing that my coolant reservoir water is "dirty" and therefore urgently requires a system drain and flush......... at a cost of £905 incl vat! Is this something I should be able to do myself easily? If so, any tips? I just think that this quote is a no no for me. Thoughts and comments gratefully received as always!