The most likely cause is the heating element has failed.
I believe your SUN0091 model has been re-branded 5891, if the unit looks like the one here, your in luck you can pick up a replacement element from ebay from here.
However if the unit doesnt look like the 5891, the parts are obsolete which makes the unit irreparable.
Looking at the cost of the new element from ebay, and allowing for £50 cost to install it (if you didnt want to do it yourself), its likely to be cheaper to buy a new unit.
For this reason, I believe the unit is uneconomical to repair, if you require anything clarified, please let me know.