Closing Fireplace Gap to Meet Fireplace Insert

[NigelSpink]

Hello
The fireplace insert i have doesn't quite reach the top of the fireplace opening (see pic) can anyone offer any advice how best to close this gap so I can fit the insert to the wall with a seal of bonding plaster?

This gap will be covered by the surround but feel as though this should be dealt with building materials and not relying on surround for sealing the fire to wall.

Under the plaster is a traditional brick archway over the fire.

Thanks in advance, any advice much appreciated.