I’m looking to use this as part of a balanced diet, the goal is to gain weight.
Any recommendations as what to eat along side Huel over the next week?
More Huel, for example? Or add some fruits to your Huel. All you need is extra calories and the most convenient way would be to eat more Huel, as you are already preparing the mixes anyway.
Gain weight? As part of a strength building programme I’d hope? 
Add more protein, like with Huel + yoghurt or Huel + milk. Coach Mark Rippetoe always includes “drink a gallon of milk per day” as part of his Starting Strength programme.
I like Huel in yoghurt and with peanut butter.
Nplima
Depends on how quick you want to gain and how much you want to spend. I eat the 2000kcal daily huel but add two 20g protein and a meal on top.
Huel doesn’t provide the NHS daily recommended calories for a male anyway as it’s 2500kcal. So maybe look into a good weight gainer or just add protein and oats which is normally the cheaper option.
Other than that you could just eat a proper meal a day onto of huel.
I make mine up with semi-skimmed milk - 1.25ltrs plus 250g of Huel gives me two shakes, and topping that up to the 2ltr mark on my blender with fruit smoothie tastes great! 4 shakes per day comes to about 3200kcal, more than enough for most people looking to gain weight