What are your exact T&C's for this?
Do they just state that the goods remain yours in title until paid for, or do they state that you can uninstall fitted items?
Normally, once you fit something then it becomes part of the property and you would not be allowed to remove it. You would have to sue for the costs.
You T&C's may be unfair and unenforceable - they are OK for a supply only contract, but inappropriate for contract that includes fitting - as once the item is fitted/installed there is technically a transfer of ownership
You are on dodgy ground legally if you uninstalled something, and the customer may be entitled to engage another to make good the structure, or have a replacement installed and then sue you for his costs
Yes you have not been paid, so you want the item(s) back, but you will have to argue the case in court.
You may find that suddenly the customer makes a complaint about the standard of fitting or quality etc, and this then immediately puts you in the wrong for uninstalling the item.
You need to get some legal advice on this.