Coming back for a long visit in May and will be doing some voluntary work in the centre. Does anyone know the cheapest place to rent / share a room? Only need somewhere very basic.

Can't really tell you as i came here through a friend, but there's this estate web, funda.nl, with houses for sale and rent. Try there, as there will be also links to estate agents et al.

Try https://kamernet.nl/

Maybe try this

https://www.facebook.com/groups/702616376420205/

It's a FB group for apartment rentals in Amsterdam. I've never used it but I happened to notice a few of my coworkers had joined, so it might be helpful.

As for the cheapest places in Amsterdam to stay, you're mostly looking at Nieuw West, Zuid-Oost and maybe parts of Noord. Good luck.