Knock when starting

[KERRSURF]

This week I have noticed on starting that my truck seems less eager to start and when it does, just as it catches, it gives a bit of a shudder and knock, I still haven't sorted out the uneven idle, I checked all the wee vac hoses. I intend to change the fuel filter tomorrow and pour in a bottle of redex when fitting the new one.
It sounds a wee bit tappety but has done for quite a while, is it worth having clearances checked?
Is there anything else I should be checking?