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My 2k DE list has some Eldar allies. I have no idea what weapons to put on the walkers though. I am thinking I need some more anti infantry but I would also like a bit of AP2 if possible.

1x Archon
3x 3 Blasterborn in Vens
3x 5 Warriors in Raiders (Dissies)
1x Crimson Hunter (lance)
3x Ravager (Lances)
Autarch & 6x Guardian Bikes
5x Scourges with either Haywire Blasters or Splinter Cannons

I already have a ton of Lance's / Blaster's and a fair few Bladestorm & AP2 shots from the Dissies. The list is pretty much balanced so I dont know what to take.

I am thinking StarCannons / Bright Lances and Scatter Lasers but what would go best with this list? My War Walkers are new, never used them before so hopefully someone knows better than I do...!

I personally do not like the Star Cannon in comparison to the Scatter Laser.

Magnets :-) honestly the best advice I can give. War Walkers are so versatile if you can go to the effort to magnetize them.

Looking at your army list, you're already taking a bucket-load of AP2, and you feel you need more?

I'll suggest Scatter Laser / Starcannon or straight-up dual Scatter Lasers.

I usually roll for scatter laser-starcannon for versatility but my war walkers are magnetized in case I want to change them out.

Scatter/Shiriken is another option. It's not natively AP2, but you have quite a bit of that already, and will get a few AP2 hits in over the course of a game, in addition to just a lot of high-str shots. If you're going up against Termies, it's not a terrible loadout, but it can still do a lot of damage to hordes. Kinda useful across the entire spectrum (short of heavy vehicles).

Well, I'd take scatter lasers all the way. Better have a Jetseer nearby with guide or prescience for increased impact.

Lets take a quick look at the various weapon combos. Keep in mind I don't have my codex on hand so most of this is from memory. Also all the odds I provide are based off a single walker's shooting.

Double Shuri-cannon: Cheapest loadout. 6 S6 shots with pseudo rending is decent against light vehicles and high touchness but the 24" range means you are close enough to eat lead from anything and everything.

Double Scatter Laser: Now you have 36" range but neither twin-links the other. 8 S 6 shots can pop light vehicles and hurt high toughness models consistently while keeping you at a safer distance.

SL/Shuri-Cannon: Can TL the S.Cannon but now we're back to 24" range. NEXT!

SL/ Starcannon vs Double Star Cannon: For when those 2+ Armor saves give you trouble. Both weapons have 36" range so you don't lose out on one or the other. I'll save you the math and give you the odds. Against a unit of 2+/5++, SL/SC inflicts an average of 2.22 unsaved wounds vs SC/SC's 2.66 unsaved wounds. While it seems like double SC is winning, keep in mind the worse the armor save of the target the better the SL/SC combo becomes

SL/Brightlance vs Double Brightlance:
SL/BL - Now we're talking. 4 S6 shots that TL the Brightlance if any of them hit. At BS 4 that's a 1/36 chance of not Twin-linking the lance. Against flyers its a crap shoot but its better than nothing. Light vehicles will cower unless they are skimmers. Skimmers will occasionally get lucky. Heavy Vehicles can be opened but with only 1 shot per Walker, you can suffer from Hit and Miss Syndrome. Inflicts an average of 8/9ths of a hull point vs AV12 and 4/9ths of a hull point vs AV13+.
BL/BL - Now heavy vehicles will quake. Less accurate overall but with double the lances, the net amount of hits is better. 2/3 HP vs AV12+

SL/Eldar Missile Launcher(Flakk) vs Double EML(Flakk):
SL/EML - This combo spells death for flyers in addition to ground based vehicles when used in a full squadron. The EML has 48" inch range so to take advantage of the SL you have get a little closer. It's cheaper than running double EML(Flakk). With 4 SL shots, you have a 51% chance of at least one of the SL shots hitting a flyer, giving the Flakk Missile Twin-link. Against land vehicles it has inferior odds to the SL/BL combo vs AV 13+. The advantage here is versatility.
EML/EML - The only point of taking these is to double up on Flakk missiles making it the most expensive option in the unit. Inflicts an average of 8/9ths HP vs AV10 and 4/9ths HP vs AV12. Inferior vs ground based targets over BL/BL

Had a game today and went with scatter / star and it worked really well. 5 man squad of Grey Knight Terminaters and a character who got back up next turn wiped out in one turn of shooting. Seemed to work really well.

I second scatter/star. It is the perfect build for your needs and as you have seen performs very well.