Bought a 2011 citroen berlingo van 7 yrs ago and recently battery would not hold charge. Took it to garage for new one to be fitted now it wont start. Apparently disconnecting the battery can sometimes cause the immobiliser which I did not even know existed to kick in. How can this be rectified? All I was given when I bought the van was 4 folding keys all the same. Is there as way to reset or bypass the immobiliser?

Many thanks in advance