Finding it hard to find specific clear info on this. Who carries out the risk assessment and what if the person needs a new chair? Is the "host" employer responsible for both? What do others do please?

I would steer you to the HSE guidance on pregnant workers, as the information in the citizen advice link is not correct. Agency workers technically 'belong' to the agency, however: All employers are required to assess whether their business creates any risk to women of childbearing age. (Management Regs). This is NOT dependent on you actually having that category of worker, or waiting until an employee tells you they are pregnant. It is part of your business risk assessment. You can never know who will come into the organisation or in what condition. When someone does then notify you they are pregnant, the HSE say you just need to review that already existing) risk assessment. You do not need to do a 'special pregnant worker' assessment. Where that idea came from, who knows. So, the agency worker should already be protected by your existing RA, having identified any specific risk (see HSE guidance on what such risks could be). You suggest that this is an office worker, so there should be no real risk, although extra attention on posture at workstation is advised. If this identifies that the chair is unsuitable, at about £150 (and you get to keep it) this seems an easy fix, not worth the aggro of trying to get the agency involved. A new chair is only called for if the existing one is unserviceable / unsuitable - there is no special chair for a pregnant worker. Just a decent one.