Gap at ceiling, plus irritating squeak, in stud wall - any solutions?

[Mosaic]

Part of my first floor consists of a small bedroom with a stud n plasterboard partition wall. All around the top of the wall a gap of about 0.5cm has appeared gradually - this is over 23 years, not suddenly! In addition there is a lot of squeaking when you walk alongside it, which I think might be related to the sole plate rather than the floorboards.
I'm about to embark on extensive renovation and would like to resolve these two issues once and for all,

Any advice most welcome.

[Count-A-Strong]

Gap: use decorators caulk or some othe flexible filler or put a cornice up.
Squeak: remove plasterboard up to about 800mm and screw down sole plate at 200mm centres.

[Mosaic]

Much obliged sir, I'll get onto it.

[The Builder]

The Count's on the case, but............ is there a supporting wall or beam downstairs under this wall? I was just wondering about settlement, that's all. After 23 years though, probably not a problem.
Sounds likw shrinkage combined with poor fixings.
I don't want to upset the Count, but I would just take off the skirting board and get at the plate that way. "Skew" the screws in. No re-plastering needed then.

[AlBags]

Nice one.. Skirting off .. Good quality Gold Self-cutting 4" screws skewed and really tight.
Jobs a goodun.

[Count-A-Strong]

Good idea builder

[Clanad]

I'd go with The Builders suggestion, with a twist however. The "skewing" he speaks of is called "Toe'nailing" here in the U.S. and consists of angling the screw into the side of the sole palte so as to enter the floor or structure beneath it. Sometimes it's difficult to know how much to angle or when to start butting side pressure on the screw to get it to "angle"
If you'll pre-drill the hole for the screw it's much easier.
By the way, you'll want to use coarse thread deck screws. At least that's what they're called here. You can buy them in various lengths and various screw "heads". Supposing the sole plate to be a 2" X 4" up'll need to go through that and at least 2" into the substructure. So... get at least 4" screws. Space them about 6 inches apart throughout the squeaking area. I'd recommend a "Star" or "Square" (sometimes called a Robertson drive) headed screw. You'll need to get the corresponding driver for it but you'll find they don't strip out as easily.
Here's a depiction of the two types of screws I described :

http://carpenterconfi...09/cc-screws2-web.jpg

I'd also emphasize Builder's other suggestion though; That being to see if you can get to the area below the structure you're describing so as to see if any settling or other damage is going on...

[Mosaic]

Thanks folks - very helpful and clear. I know there are no foundation or joist-related issues, and it seems no coincidence that in other parts of this extension I've had problems with addled plaster. Looks like the guy was in a hurry when he got to this part of the build!