Going by the avocado tint and the presence of 4(!) fixing bolts I'd date that to the very early seventies.
Not saying it can't be fixed but IMO retirement would be the kinder option.
McAlpine products are generally excellent, better engineered and more solid than cheaper alternatives. Never saw a bad one.
Can't imagine what went wrong in this case; dare I suggest user error?
OP
The manufacturers' installation instructions will give the answer to the issue of the correct orientation of the tappings.
I'd be surprised if they did not specify tappings uppermost on a horizontal unvented.
^^
https://assets.publishing.service.gov.uk/media/66f6c6ce3b919067bb4828cc/ADG_with_2024_amendments.pdf sections 3.40, 3.41 , 3.42
OK, I'll amend my post to stating that G3 qualification (and scheme membership) is required to notify under one of self-certify schemes.
Other...
"To reach World Heat Cylinders (manufacturers of unvented hot water systems) for technical support, sales, or spares, you can call them on 0161 343 8610"* (AI result)
I think your installer has stuffed it and seriously doubt that he holds the mandatory G3 qualification.
*EDIT THIS THE WRONG...
You face forward (towards the door) on a squatter.
A proper Indian toilet does not have a toilet paper dispenser like in the picture above.
You use water from a little cup, and there is a strict etiquette about which hand and which finger does what! All a bit disconcerting to the first-time...
Escalation to gas leak detection may not be required.
There may be an issue an experienced plumber can spot in five minutes if he's given the space to look around.