Is there any trick to checking the oil levels?  Or do you just have to drain them out and refill on a regular basis?

You've sussed it. 

Urf.  Would be nice if there was a dipstick or sight window ... or even if the gearbox drain plug exited on the bottom ...

I dip a cable tie in.....

The clutch oil filler is right above the clutch itself, but I'll have a bash once the right amounts of fluids are back in it, maybe there's a way

The clutch side has a bolt you can take out to check the level (so if too much oil in it will dribble out). 

 

Thanks, I'll have a look!