The Arch Company probably just includes in your agreement that you must make your own enquiry concerning business rates.
Check on the
VOA business rates site, search for similar arches nearby and look in the detail for the current valuation find "how the valuation was calculated"
That will give you a base rate per square metre. From that you can work out the Rateable value and the Business Rates will be approx. 50% of that figure per year.
But you may find if it isn't on the VOA web site it may not be registered and so you may not be charged.
Also for a small business with only one premise if the Rateable Value is less than £12,000 there is
"small business exemption"