Mine is a bit gritty, anyone know if it's possible to get at the internals?

It is possible to get into them with a pin spanner. I got mine apart once but won't bother again. So many tiny ball bearings and shaky hands is not the best combination.

I don't know which Shimano frewheel I have on my cruiser but I took it apart using a pin spanner like above. I didn't remove any bearings though, just sprayed it through with brake cleaner loads before putting a grease / oil mix back in and it's been fine. I'd do it in a contained space though as Mister P mentioned, the bearings are tiny.

Thanks for the advice, I'll have a look around for a pin spanner and start to tinker.