First of all you need to appreciate that if the company is in financial difficulties, it is possible that you could end up with nothing if, for exaple, they go into liquidation. In your shoes, I would proceed as follows.
I would write a letter stating that you would like either delivery of the building or a refund of the deposit by a certain date (2 weeks, perhaps?) otherwise you will be taking legal action. You should keep a copy of the letter and ask the post office for proof of posting (or recorded delivery). If nothing happens, you should start the small claims process.
http://www.moneyclaim.gov.uk/
If the company does go into liquidation, you could contact the liquidator and try to take possession of your building which, I expect, will be currently on the company's premises. If you don't do this, you might get a few pence for every pound that a company owes when it goes into liquidation.