we are having an extension and our next door neighbours have informed us that they want us to pay for an structual engineer to look at their property are we liable for this cost
Almost certainly - do you have an architect? If so thry should sort it out but you will have to pay, when we had ours built end of last year we had to get the neighbours to sign all manner of docs, including a party wall agreement, with all the legislation nowadays I wouldn't entertain building anything without an architect - they take all the flack (allegedly!)
If you are building right on the boundary line the Party wall Act applies and your neighbour has the right to appoint a surveyor (not a structural engineer) to ensure the what you proposing to do is OK. Didn't your builder tell you about this?