We've recently rendered the back of the house but the bricks were in a bad way and the finish is not very smooth.Is it possible to use high build to hide the imperfections but with a smooth finish?

pay cheap pay twice ,nothing can make rough render smooth ,unibond and apply a proper coat by professionals at the right price

It wouldn't matter what condition the bricks were in if it was rendered properly. Put a request on Post a Job for a (local to you) builder/plasterer who specialises in external rendering to visit you and to give you some professional advice but it looks like to me it would need going over again with another coat. Carl.

seal old render and apply a new coat of render ..good luck

need to apply another coat

Regardless the condition of the bricks were in at the beginning, It appears that you will need to apply another coat as this has not been done correctly initially. I would advise you hire a skilled plasterer but obtain quotes allowing them a site visit to view the job.