Surprise Knockdown (goes well with 15% crit chance) - if I knock enemy prone I have combat advantage next turn ?
You have CA against prone enemies, but chances are that any enemy that you knock prone will stand up on its next turn. If you knock an enemy prone and then use an Action Point, you can attack them again before they stand up. Otherwise, this will just force your enemy to spend a move action to stand up. Maybe useful for a rogue, because it can pin people down but chances are that you won't get much use from this feat.
Slayers Action (MA) - looks very good.
The only thing I can find named "Slayer's Action" is a fighter paragon path feature from Martial Power. There is no way for you to get access to this, since you're going to be taking Daggermaster as your paragon path. There is no other way to get paragon path features from another paragon path. If there is another feat or power named Slayer's Action, then ignore me because I don't know what I'm talking about.
Fast Runner - is +2 to speed going to be useful ?
Maybe? This depends a lot on your party composition and DM. If you have defenders and controllers to slow and pin enemies, then it won't be important. If you play a lot of small interior rooms where there isn't much space to run, then it won't be important. I wouldn't take it unless you can't find anything else worth picking up.
Improved Initiative - is +4 initiative useful ?
Yeah, it's pretty useful. Some people take Quick Draw instead, since it also provides additional benefits about drawing weapons. If you throw a lot of daggers, then this might be useful, until you get a magic dagger. The initiative bonus provided by Quick Draw (+2, I think) doesn't stack with improved initiative, so take one or the other.
Press the Advantage - to keep CA if you crit seems very good, is that any good as my build looks like it will not have enough Charisma, or is CA usually gained pretty easily ?
Rogues have some good ways to get CA without charisma. In fact, Bluff is only one way to get CA for one turn once per encounter. Rogues have better ways to get CA, like putting on conditions (stun, blind, and daze), through stealth, and with flanking. You can still be an effective rogue without Charisma.
Secret Stride - moe at full speed and hide, useful or will Rogue utilities be enough
Make sure you really understand the stealth rules before you try to make stealth an important part of your character. Those rules are still a bit of a mess, and tough to understand. At least, I've had trouble getting them down. IMO, you can still be a very effective rogue in 4e without using stealth.
Brutal Wound (MA) - is this going to be good ?
I'm a big fan of ongoing damage, so I like this. Other people may find it less cool, but I think it's a solid feat.
Lasting Advantage (MA) - a level 21 must have for Dagger Master ?
Looks like a very solid feat. Daggermaster = More crits. Lasting Advantage = More CA. Should let you do your thing in style.
I am thinking using Hand-Crossbow for Ranged sneak attacks before the main battle commence's ?
Throwing daggers works well too, but I don't think the range is quite as good. Remember that you'll get an extra +1 to hit with daggers. At any rate, you don't need to worry about keeping track of minor actions as much.
Daggermaster Action - why spend an action point to re-roll an attack when you just spend an AP to get another attack, whats the benefit ??
Byronic nailed this one on the head. Rerolling lets you take another crack at landing that super-important daily attack power.
What sort of Utilities, Encounter Powers work well with a Brutal Scoundrel Rogue 20 Dex, 14 Strength, 13 Con.
There are a lot of powers out there, here are some that I like from the first few levels. Also, a few powers are dependent on being trained in certain skills. I don't know what skills you have, so I've ignored those entirely. Enjoy.
Level 1 Encounter: Sly Lunge (MP). Deals extra Strength damage and the baddy grants you CA for next turn. Can only target baddies who are already granting CA, but if you get it once, you can stretch it over two turns without having to do any work... if you hit.
Level 2 Utility: Adaptable Flanker (MP). Gets you combat advantage if anyone at all is adjacent to the baddy.
Tumble (PHB). Lets you shift 3, which is almost always enough to get into a flanking position against a medium creature. Can also get you out of trouble if you need to escape.
Level 3 Encounter: Setup Strike (PHB). Target grants you CA until the end of your next turn. Effortless CA.
Nasty Backswing (MP). Triggers on a miss from another attack. Automatically grants you CA. Allows you to shift 1. Deals strength damage. Free Action attack.
Level 6 Utility: Vexing Flanker (MP). Shift into any space adjacent to an enemy when an ally moves adjacent to the enemy. Automatic CA for your ally, and for you unless the badguy moves after you. Immediate Reaction, so it doesn't use up actions on your turn.
Sidestep Stance (MP). +2 AC against a single enemies Melee attacks until the end of the encounter. Good to improve your defense, if you're getting dropped frequently. Daily power.