I received my new crankset from Merlin cycles minus any fitting instructions.

I downloaded these from the Rotor website but to be honest they aren't to clear.

As I've fitted many chainsets in the past I installed these on the bike. Problem I have is that under the recommended torque (35-40nm) for the tightening bolt the cranks do not spin. I have to back it off so it's just tight at about 25nm.

The bottom bracket a BSA30 came with just the cups and a 0.5mm sleeve. Is there anything else I should have?

Also doesn't help that the splines on a Rotor BB differ from Shimano, FSA or SRAM and require there own type of tool that of course is not supplied as the items are OEM.

You need to turn the big pre-load nut on the left hand side all the way in towards the crank arms. Then install, and tighten the nut until there's no play and the cranks still turn properly. If the pre-load nut is already turned in, see if there's any small spacer rings you can remove. There's usually one 1.5mm spacer against the pre-load nut that can be left off.
The tool is normally supplied with the BSA30 box, but as you say, not if it's OEM.

I'm pretty sure the tool for that BSA30 bb cup is the same as the Zipp Vuma cupset tool and FSA megaEVO (not megaexo) tool, so ask around.