bolts at side of rotary blades will not loosen?

There are two bolts one either side of the rotary blade on my Qualcast 35S petrol lawn mower. I understand they can be turned to adjust the blades themselves. One will turn while the other will not thus the cartridge is now lob sided. If I could release the bolt and adjust it to level the blades the mower would cut while at the moment it will not.

I live in Liverpool and would like to know if there is a reccommended repair service in the locality unless you know of a way to release the bolt.