The clause you are referring to is sometimes called "retention of title". Our Ts & Cs have it - you can find it in paras 6 onwards on page 2 of [DLMURL="http://www.williams.uk.com/creditapp%20for%20web%20jun12.pdf"]this link[/DLMURL] if you can be bothered with the small print.
The problem with repossession is two-fold.
If the items are incorporated into the fabric of the building, then you can't legally remove them. I had a case once where a site contractor went bust, and we went on site with a load of lads and vans and recovered a lot of gear. The definition of "incorporated" became a bit vague in all the hustle and bustle.
Secondly, you can't cause damage entering a premises, or as a consequence of removing something.
In these days of mobile phone video, its a lot harder to just grab everything and run, and deny it later.