Chris,
Firstly - if the report comes back as good - what do you intend to do? You've paid out £160 or £260 and are you likely to get much cash off the price?
Would you be happy to pay the full £9K if it's what the seller says it is?
If so that's the right thing to do I guess, but as others have mentioned, you don't actually know at this stage whether the seller will move on price, especially if he thinks you're keen after paying out for an inspection
If you're looking for a 'bargain' I reckon it's not
that cheap personally. I've seen proper CLK55s down in the south with similar mileage, better spec (COMAND etc) and in good straight condition for £2k more than that.
Good Luck,
Will