Every time you buy or sell a share, the transaction is settled electronically by a company named STRATE. Without this settlement the transaction could not take place.

For providing this service, STRATE charge an electronic settlement fee on each transaction.

In order for us to reduce transaction costs for our clients, we do not charge them the full STRATE fee for that transaction. 

Instead, we charge our clients a flat fee of 0.075% of the value traded without any minimums. This is so that we can recover a small portion of the fee that we have to pay to STRATE for settling the transaction.

By doing it this way we are able to lower your transaction costs and give you greater value in your investment.

There's a full post on the fees charged to Investors as well as an attached cost profile available HERE.