So, the interest line is the total interest you'd pay if you made 48 regular payments. i.e. if no pre-payments, then fine, although still unusual to be written that way.
What it reminds me of is my home mortgage with Wells Fargo. If I paid more than the regular P&I payment, on say Nov 1, rather than immediately reducing my principle, they applied the extra to the payment due on Dec 1 and if my prepayment was enough, keep pushing it out.
I had to call twice to get them to recognize the timing of the payment. Prepaying is quite common these days, it was surprising that a firm like WF would assume people were just paying a later payment a little early. The default should be as a prepay.
That said, look up the agreement to see if no prepays are allowed and call them. They could simply be doing something along the lines of WF......basically trying to screw you a little.
I see it is in GBP's. In the U.S. not allowing prepayments or having prepayment penalties is highly discouraged, with the main market being mortgages (Fannie and Freddie will not buy mortgages that do this).