Hi flame,
re your comment:-
"Have been trialling a bit of the customer/client thing too, actually. Well, not that exactly - but, for e.g. when my Dad makes a rude comment about me in the guise of a 'joke' instead of responding by trying to defend myself as I usually do I just gaze at him in an uncomprehending fashion. It's really pissing him off".
"I just need to get to a place where there words and actions can't hurt me anymore. Hate myself for feeling annoyed/upset by the things they say".
I would not just be annoyed, I would feel very angry with both parents for making me feel that way in the first place. You seem very much in FOG (fear, obligation, guilt) with regards to them which is not surprising either.
I would cancel their visit nearer the time; cite family illness or some such. BTW who invited them?. Raise your own boundaries a lot higher with regards to these people. I would also suggest you post on the "stately homes" thread on these pages. Read "Toxic Parents" written by Susan Forward.
It is NOT your fault they are the ways they are; you did not cause them to act that way. Remember too that "no" is a complete sentence.
If they are too toxic for you to deal with its the same deal for your vulnerable and defenceless children as well. They were not good parents to you and are likely to be poor grandparent figures to your children as well. I would keep them well away from your children, narcissists in particular make for being deplorably bad grandparents.