The flavor is already..

   in the mix. Its basically instant pudding, just add milk, whisk and wait 5 mins to thicken. It comes in a variety of flavors here in the USA...butterscotch, chocolate, vanilla, pistachio, coconut, lemon to name a few. The ones here come in 4 or 8 serving size packages. The size that your recipe requires is a 2-serving package of instant pudding, so for us to make it here in the USA we would just need to use one 4-serving package of instant butterscotch pudding mix. Our pudding here is not as firm as a custard.

You can probably order the mix online through some British export grocer. But, any instant pudding mix should do the trick. 
Hope this helps.  

Reviews and Replies:
1	ISO: packet butterscotch-flavor dessert whip
	eggy/m'sia

2	Its powdered custard/pudding mix (nt)
	Kim, WA

3	Thank You: kim, how do u get the butterscoth flavor (nt)
	eggy/m'sia

4	Post the recipe, we'll try to help
	AJ in MD

5	The flavor is already..
	Kim, WA

6	Recipe: Is this similar to the recipe you have?
	Kim, WA

7	Butterscotch Pudding
	bme aust

8	Recipe: Buttersctoch Squares (using pudding mix)
	eggy/m'sia

9	eggy/m'sia
	Patsy,La

10	Feel free to ignore...
	AJ in MD

11	Dear Eggy
	Betsy at TKL

12	Thank You: dear AJ & Betsy
	eggy/m'sia