Hi Stephen,
You probably refer to the Assurance of Support bond. It is normal to take a long time between lodgment of the bond and visa grant. It is best if you let your case officer know that the bond has been lodged and email a scanned copy of the receipt. This may speed up the process a little. Also check the dates of your police clearances and medicals. Most of those documents are only valid for 12 months. Your application has been pending for 18 months. You may check if a request for new set of those documents is coming.
Dessie