Kevin_Gravill,
You should setup a merchant account for that. Click on ** Banking**, scroll down until you can see Merchant Account. That is the place where you new Sumup account will be. Go back to the top and click Create Bank Accouint. Call it Sumup. Have a look on the link below. It is about paypal and gocardless but the setup is the same.
When you have created that account, open that account by clicking View. Click Input New Transaction Fill all fields, Date, Discription,in, £100 and press enter or click add. The line you created has a Tag field at the end. Click it and tag it to the customer who paid the £100.
Click Input New Transaction again and to the same with thw Sumup fee of £1.69, but this time you have to choose out. When you are done click the Tag button again and link it to/ or create a invoice for the £1.69 for your “Supplier” Sumup. Repeat with more payments if necessary
When Sumup transfers the money to your bank account. Tag it as a transfer between accounts and choose sumup.
You can also upload a csv file, which you have downloaded from sumup, to your sumup account in quickfile. This would make a few things easier and you need only to do the tagging part.