Thank you all for your help and advice on this one.
I think the text books will reinforce what they are being taught in school. Also its always a good idea to have something to refer to if they get stuck on a particular topic.
Their school has subscribed to Mymaths and the boys really enjoy completing homework in this manner. They seem to be doing very well in all their subjects for the minute apart from Maths.
Apparently the Maths teacher told one of the children "Are you sure you are in the right school" and "I am beginning to think whether you should be here". After speaking with a number of parents from other years this is a continuing problem. I have not made a big deal of it at the moment and am encouraging and building confidence in my son in this subject.