I got offered 6% without too much effort (but that was with no part-x deal), my dealer said he's still amazed by the number of people that don't even bother to ask for a discount!
I'd have a go at getting a better deal for the part-x, there are always other garages you can get your servicing done at if they are not kind to you.
At the end of the day though, like a lot of things in life, if you don't ask, you dont get! I'd say you've lost some leverage by already ordering but there is still the servicing aspect to barter with. What have you got to lose by asking?