Rate of pay query

Hi, I've been freelancing in various roles for many years - originally in book production management roles in London, and now for the last few years I've been contracted as an administrator/manager for a group of business angels.

My role is expanding so much that I don't have the hours to do it all myself anymore, and I'm looking at getting someone to work with me, taking on the basic event management work. Organising event calendar, booking venues (from an existing list of venues), sending out invitations and logging attendance etc - for approximately 20 events a year. Each event will have about 60 invitees, and 20-30 attendees. I'd say it's about 10-12 hours work a week, working from home / your own laptop wherever you are, in your own time. A few site visits (Cambridge colleges mostly). School holidays things drop to 3-4 hours a week. Will be asking the person to invoice the organisation monthly for hours worked, submitting a simple timesheet.

The work is really straightforward - I'm thinking anyone with good interpersonal skills, basic administrative experience. I've got systems all set up in place already. Would probably suit a mum of small kids who has stopped work, but wants just a few hours work a week. They would obviously need their own computer, use of Word, Excel, email...

How much would be a fair hourly rate? I'm thinking £12-15...?

Any advice would be very welcome.

Thanks
Oonagh