Yaris GR4 Misfire/Judder under acceleration

Hi,
I'm keen to hear back from anyone who is getting a very short duration misfire/judder under hard acceleration (3k to 5.5k revs) in gears 1 to 4. Its totally random and can go days between it happening.
Its a 71 plate circuit pack, daily drive with 19k miles, no mods and not tracked yet. It's been into the local dealership three times over the past four months and they have so far been unable to identify the issue. Its occurred when they have driven it, but it happens too quick to throw up a fault code in the workshop and apparently the Toyota central technical team need a fault code to authorize a warranty fix.
They initially questioned the fuel quality but this got ruled out after I went through a full tank of Shell super and still had the issue. They then changed out the plugs and again this didn't fix it. I have pointed to the coil packs as a potential cause after reading some of the posts here, they are yet to confirm if they will replace them.
The car is always warm when it occurs with traction control, etc on. I recently did 2k miles doing the NC500 and Kielder reservoir and the misfire came in only three times, always under acceleration.
If anyone has the same issue and any suggestions on the fault finding and how to get a warranty claim through Toyota without a fault code I'm all ears.
The dealership are trying to help, but its now doling my nut in!

Hi, 

a bit too late and from someone that hasn’t got any experience with Yaris gr4 but generally speaking misfires that aren’t detected by the car ecu are most likely from fuel air mixture or coolant entering the combustion chamber. 
In UK the petrol quality is appalling and because it’s a Shell or other top tier garage and fuel means nothing and the petrol itself could be pure garbage. 
First thing to do when you experience these symptoms is to change the garage where you fill up. Run two tanks of different fuel and see if there are any difference. Avoid places with cheapest petrol prices no matter what brand they are. My recent discovery was that my car had cold start knocking caused by misfire and happen only with some particular fuels from local Tesco cheap 99 e5 ,  which I really liked but the cold starts were horrible. I have used more expensive Tesco e5 99 and no more knocking on cold. Also changing frequently garages I found the car drives so slightly different, engine is more or less responsive and efficiency up or down. When I like a place I stick with it until I noticed again any abnormalities. Then change the garage and fuel and all back to normal.