The free listings magazine 'Jazz in London' will provide your visitors with plenty of possible suggestions. It's available from most jazz venues but you don't even need to leave your desk to get it, because you can download it, as a PDF document, from here: http://www.jazzinlondon.net/JIL/JILfrontpage.h tm
Check out this place - Quecumbar in Battersey. Has a site www.quecumbar.co.uk . Took friends there - quite a good place. I read some blogs about it before hand and it was much as described. Food good, wine very reasonably priced, ambience excellent. Have to pay for bottled water - no tap water. Would say it is worth a visit
The vortex in dalston is the most famous in north london.
www.vortexjazz.co.uk
Haven't been here but it''ll be a damn sight cheaper than ronnie scotts.
And has anyone mentioned the 100 club on oxford street? for jazz during the day.