This might be the point you are making but if you built it to their specification and their specification was for it to be mobile, how can they now say that it isn’t mobile?
Just because it has to be lifted by a crane does not mean that it’s not mobile. Portakabins and shipping containers have to be lifted by a crane but nonetheless, they are mobile.
Who told them that it must comply with building regulations?
Do you have something from a survey which says that it does not?
How much is it cost?
Apart from the leak which you would have found had you pressure tested, what other issues are there with it?
Your question seems to hinge around whether you have to actually instruct an expert. Not at all. There is no requirement for you to have expert evidence. What you have is an order that allows you to get it. If it said that you must get the necessary evidence, that is a different issue.
However it is always advisable to get the best expert evidence that you can.
Unfortunately, for some reason, I cannot open the link although I can tell you that the wording of the order is quite normal.
That appears to answer your question.
Can I clarify anything else for you.
If so, I’m probably going to need the rest of the information that I’ve asked for. Thanks