There should probably be an extractor bolt which you undo that bolt against - thus pushing the crankarm off.

My Rival chainset lost one and a replacement cost me £14 IIRC.

Still cross about that. Shimano next time.

Bother, I thought that was going to be the answer.
Thanks for the help though.

Surely you just need a crank puller?

edit: just realised no thread in the arm so would need a normal puller or tap it off?

I managed to get it off with a crank puller. I reinstalled the 8mm bolt and then backed it off bit by bit.
Only problem now is that is that I want to sell the cranks so I now need to track down a replacement bolt.

As above, I had some Rival cranks with a cover plate which keeps the bolt captive in the crank arm. The bolt works as both tensioner and puller when backed off.

something like this ... ??
http://www.fatbirds.co.uk/170544/products/sram-crank-arm-bolt-m15-m26-alloy-self-extracting-gxp-black--1-pc-.aspx

Just like that.