I am not sure what the Wilko kit instructions recommend but my advice is not to add any more than 500g of sugar/dextrose to make up the wort and preferably less than that, with the balance being malt extract, not 'brew enhancer'. I also advise that you don't brew to 23 litres. Around 20/21 litres is fine. And the reason, you only get 1.5kg LME in the can and that will produce 'thin' beer, more so if you use lots of sugar. That's a lesson you don't need to learn by personal experimentation, just take my word for it.