Q.1.   No, Speedy Hire Plc is only one company, therefore the singular verb "is" is correct.
Q.2.   Using "we" and "our"  you or they are really referring to people there,  and then you use plural to match the plural "we".   "our"
Q.3    "Sanderson Tiling are" is incorrect, should be singular.   It is like a compound noun, like the noun "crowd", or "group", or "flock" as in flock of birds, all these compound nouns when in the singular form, use singular verbs.  You can, of course, make them plural, like when you refer to more than one crowd, which then becomes crowds, and then you use the plural.