Can having a hiatus hernia and having oesophagitis cause severe chest pain and pressure discomfort?

Definitely: If you are not on a proton pump inhibitor (such as omeprazole), you should be. But yes, hiatal hernia can cause very severe pain.

Yes it can however..: You should have your doctor make sure that no heart issues are causing the chest pain before exploring the hiatal hernia as the cause.

Maybe: If the hiatal hernia is large enough, it can cause chest pain. These large hernias are also referred to as paraesophageal hernias, and are diagnosed with upper GI contrast x-rays and/or upper endoscopy. Hope this helps!