Risk Assessment for volunteer staff member, wheelchair user

Hello there,

I'm the volunteer coordinator for a charity where we have a number of charity shops in Yorkshire. We've had an enquiry from a lovely member of the public about helping as a volunteer in one of our shops, and she is a wheelchair user. So what I'm needing to do is have the shop risk assessed for a volunteer who uses a wheelchair. We have a duty of care for our volunteers that's different to our customers as it would include use of our onsite toilet facilities and assisting with sorting donated stock.

We also have a duty of care which is slightly different to our paid staff. 

I wonder whether anyone on this forum could advise me in how and who can risk assess this kind of activity? Our own health and safety team have asked me to look into it.

Thank you!