When re-potting or digging in the soil I keep finding these fat "grubs" up to about 1cm in length, they have a whitish-grey body with an orange-brownish head part. Are they a "bad guy" or a "good guy"? Have they got anything to do with slaters as I have lots of them (complements of mulch!).
This is a bad guy Gill. What you have been finding are cockchafer grubs, they are the larvae of the African Black Beetle and overwinter in the soil.
They eat the roots of potted plants and turf and can be a major problem if there are plenty of them.
They can also be difficult to control. Carbaryl can help with control, you will need to read the instructions as to when and how to use.
Sanitation is probably your best bet though - especially as you are also having problems with slaters.
Remove any litter from the soil surface, use clean potting mix in your pots, avoid spreading infested soil around your property and just try to kill the buggers! This should help with the slaters too, remove any places these guys can hide and maybe find a new source for your mulch.
Watch out for them in your compost heap too, this is a favourite place to lay eggs where they can hatch in perfect, warm conditions. its good to run compost through a sieve before using it then you can pull the grubs out. I toss them on the path as a tasty snack for the birds.
P.S. thanks to Agriculture Victoria for the images Below