I took it out so I could paint the housing up. I believe it can be done in situ and removing the bonnet will help loads.
If, when you remove the matrix, the sealing foam is all shot, fix it with foam camping mat. You want all the air from the fan to go through the matrix not escape round the sides or out gaps in the casing. It's surprising where air can leak out (where the pipes exit is an obvious one) and what a difference it makes to the heater when it's sealed up properly.