Hi Adam - I'm not sure how a new sole trader's list of account codes appears these days but, if you look at yours (Settings>Accounting>Account codes) you should see a couple down the bottom called something like "Owners equity" and "Drawings".
The idea is that anything you inject into the business is put to the first account (eg your payment above) and then anything you draw out of the business is put to the second account (eg when you want to take the money back out).
At the end of the year, if you look at the balance sheet report, you'll see the two accounts set off against each other and any profit added (or loss deducted) from the combined total, to give you a total net sum for what you have invested in the business.
In other words, If you put £100 in at the start, have £5,000 drawings in the year and make a profit of £6,000, the balance sheet report will show a total of your "equity" as £1,100, which will then be carried forward into the next financial year as a starting balance.