An experienced bookeeper is probably different to a PA? But I appreciate you might want to combine both activities within the one post being a small co.
If you are looking to pay somebody a salary I would have thought £25-£35k gross pro rata would get you somebody with a bit of experience. You can pay less, but it might limit who would apply. I think the hours will make a difference too, if you need them in everyday that is c£7 a day tube fair (as an eg) that they need to cover from the income.
If you are looking to outsource to a self-employed type I would have thought the rate would be around £20 - £35 an hour.
This website is good for part-time roles many of which are based in London to give you an idea - www.timewisejobs.co.uk/