Thanks for the prompt reply.
I put the problem to my friend who put me on to the L298 bridge and he gave me the cause of the bridge not working at 24v.
I did not realise that if the 12v jumper has been removed you must connect pin 6 to a 5v supply as input to the chip.
Looked through lots of web pages on the L298 and none of them mentioned this.
All working as it should now.