Lightweight heater is rubbish

I recently fitted a replacement heater unit to me 1974 S3 lightweight. It went in OK, the matrix gets red hot and the fan works on both speeds but it just doesn't seem to blow hardly any hot air out of the outlets.

Its the rectangular shaped one that sits below the instrument panel in the centre of the bulkhead

Have I installed it wrong or are they just useless like that? It's a pain in the arse in this cold wevver

mine gets roasting, but I do have a V8. If the matrix gets hot I would assume a fault with the fan. how fast is it turning? their not great, but their not bad either.

G