Here's the heater as it came out of the vehicle:
The heater appeared to be working okay, so it was just a case of taking it to bits, cleaning it up and putting it back together.
In bits:
And back together:
Mrs Jonesy thought I'd bought a new one! Praise indeed, I reckon. The matrix inside was treated to some kettle descaler, but apart from that it was in excellent condition. The "draught excluder" inside was rotten, so I used some camping mat to make a new one - it just ensures that the air from the fan is blown through the matrix and not straight out past the fan motor.
I was rather chuffed that I managed to peel off and retain the original "Smiths" stickers, so these have been stuck on again. I discovered that these heaters are still available new from L.R.Series - Given that my "restoration" cost me about £15 for the paint, I don't think that's too bad at all!