Sunny customer here! Though never gotten an email response out of them... and they say the majority of their customers don't pay them back
Have you tried making a payment OP and then seeing what options you are presented with on the drop down option on your online account?
I find when I make my payment, for a short time immediately after, it does allow me to pick an option to pay less although the 'debt' sticks around that bit longer, it is one way to deal with it without any further hassle I am just referring to mentally, (*circumstances dependent of course*)
Failing this yes you need to re-contact them as you say they have been okay up to now, hope you get the action you need x