Hi,
That's a mk1 Metro heater so '80 - '84 it's the easier of the two to diagnose being simple rod adjustment on the right hand side of the heater and there are less places for the air to escape. The key is to make sure there are no leaks between the blower motor and the heater intake so you're not losing air also make sure the intake blank of the right side of the heater is intact (on a LHD car this blank is on the other side) I have seen them broken or missing before now. Also check whether there is a gap between the bonnet and the blower motor on my convertible there was a reasonable gap so I was getting engine bay smells and low heater flow the easiest test is a thin smear of grease around the blower motor rubber shut the bonnet fully then open it to look for the tell tale grease mark on the bonnet. If there's no grease on the bonnet then it's not making contact so use a section of door rubber around the hole in the bonnet to take up the gap. The other thing to check is if you have good flow from the centre vents (from memory they only blow cold) but poor flow from the demisters may mean that the feeder pipes from the heater to the vents have been pinched where they come up past the dash support by the bulkhead or that the have a poor seal or have come off the vents.
Don't expect miracles but you should be able to have piping hot air but flow will only ever be acceptable.
Cheers,
Mike