Thanks for the suggestions, guys - with the past few days of nice weather, temperatures in the tanks have all been bang on target, consistently, but I'll definitely need to look at insulation.
Brenton, it's a rental, so unfortunately I can't do too much that would permanently alter the place. Drafts under the floor is likely our main culprit as directly below is our cellar and there's only floorboards between us.
I'll look into how to insulate a part of the room, at least - currently some of my tanks are in a wardrobe... which sounds weird but it looks cool when it's all open (and protects the tanks from over zealous dogs when I'm not here. There may be a way to ensure the wardrobe itself stays insulated.