One of our sites rents a small mill, which is situated within a farm. We are needing to change the fire escape route as its a hazard. Would it be the landlords responsibility to cover costs, or our Company...I've heard differencing opinions, so would like some clarification if possible

thank you

In the context of your question it cannot be properly answered because the main legislation which deals with fire safety (RRFSO) does not define explicitly who pays for fire safety, improvements, etc. You could check the terms of lease in respect to maintenance/repairs and who pays for that, otherwise you may have to come to an agreement with the premises owner.

In short, whoever is in control of the premises, assuming a workplace, will be deemed the 'responsible person' starting with the employer.

Meaning of “responsible person”
3. In this Order “responsible person” means—

(a) in relation to a workplace, the employer, if the workplace is to any extent under his control;
(b) in relation to any premises not falling within paragraph (a)—
(i) the person who has control of the premises (as occupier or otherwise) in connection with the carrying on by him of a trade, business or other undertaking (for profit or not); or
(ii) the owner, where the person in control of the premises does not have control in connection with the carrying on by that person of a trade, business or other undertaking.

The real question however is why there was no H&S and E duty of care questions asked before taking on this lease. The answer will ultimately lie in the lease terms.