WEM Mk4 Copicat - low volume output

[Bushbaby]

Ive taken some pics that will give you an insight to the unit. I have felt the heads I cant say that I can feel any lumps or bumps although at the very bottom of the a couple of head it is possible to feel a wear line(?). I assume that these are BSR type heads as they have an adjustable metal shroud. I have seen replacements for £35 per head but this does seem very expensive??