Hello, my name is Jim and I am a qualified lawyer happy to help you today.
Until you make a formal claim against them and sue them, they can withhold the documents. The best way forwards would be to prepare a letter of claim which complies with the pre-action protocol for construction disputes (content of your letter can be found here: https://www.justice.gov.uk/courts/procedure-rules/civil/protocol/prot_ced).
Once they respond to you, assuming they deny liability, you can threaten them with an application for "pre-action disclsoure", which means you ask your local county court for an order before main proceedings start to make the company disclose the documents to you (the plans). The reason the court would allow this is that disclosure is desirable before a claimant involves the court in substantive proceedings as the documents may help avoid litigation - and narrow the issues between the parties.
So, the only way you can access those plans in my view is to send them a formal letter of claim. Wait for their response - if they deny liability then they have to disclose documents in support. If they still do not send you the plans then an application can be made for pre-action disclosure which costs £255. That fee is recoverable from the defendant if your application succeeds.
You could instruct a law firm on a no win no fee basis to handle the whole lot - the letter of claim, application to court, suing for damages in the main action. If so they try http://www.slatergordon.co.uk or http://www.irwinmitchell.com, both of which have expertise to handle your dispute.
I hope this helps - if you can please accept the answer and give me a 5 star rating (the top right of your screen & then click “submit”), I can answer follow up Q&A's at no extra charge and I will be credited for helping you today.