Very Quick Harlequin Rasbora Deaths.

When you say that all tests were Ok, did that include nitrates? Did you do the very large water change required after cycling?
Another possibility is that the water parameters in your tank are very different from those in the shop, did you check their pH and hardness and compare to yours?

Sorry to hear about the sudden demise of your new fish, after you had done so much to make sure everything would be ok. 
Theory-wise I can't add anything to what I have already seen the others post.

Thanks Little Fish, it was extremely irritating and dissapointing.
Sorry Richard did you German hardness or general hardness? I have general hard water, not sure about KH though.

Sadly not, I lost the receipt but they did say "if it happens again we'll sort you out"

But I did only get the original 6 for 1.99 thanks to a dodgy till 

I am know thinking it's karma though 
lol

I'm sure the guy said 6, it was a while ago so I could be mistaken.

Ahhh that could have been the issue, I left them for about 15/20 minutes once I got home.

There's no obligation for the shop to refund for fish which die shortly after they are sold. Especially in a case such as this, where 5 fish died quickly with no apparent sign of disease, the cause is almost certainly some form of shock, whether temperature, pH, nitrate or osmotic (caused by different concentrations of salts in the water). This is the responsibility of the purchaser, not the shop.

In terms of getting compensation or replacements, the following advice copied from Practical Fishkeeping is as good as any :

"Test your water immediately and take a sample from the tank straight away. Half a clean jam jar’s worth of aquarium water will be more than enough for any self- respecting retailer.
The first question you will be asked is what the water conditions are in your tank. If you don’t know you don’t have a strong case. Equally, if you don’t know but refuse to provide a water sample then it’s unlikely that any retailer will help you.
If testing reveals your water parameters are unsuited to the fish you purchased, or if you added it to a tank where ammonia or nitrite was rife, then you need to accept that you were responsible. Only if explicitly informed by the retailer that the fish would be fine in your conditions do you have grounds to hold them accountable."