Definitely seek legal advice. If the freeholder hasn't followed the The Torts (Interference with Goods) Act 1977 by serving the correctly worded Notices and following the legal process, you will have a claim against them. I would take a picture of the junk that was left behind (if it is still there) as evidence that this cannot have been solely to eradicate a fire risk. If the garages are not adequately fire proofed, they may constitute a risk and it would depend on what rights you have to occupy the space so a lawyer would be best placed to advise.