OK - I think I have something that might work.
The "murder walk" will be a saunter with about 10 stops starting and finishing at Keble. So no guided tour this time - just a bit of exercise and (I hope) a laugh!!
All being well the hole thing should be over in 90 minutes or so. (If it's pouring with rain we might be able to do the whole thing indoors in a pub if you preferred.)
At the start and at each stop I'll tell you about a murder investigation and you'll have a chance to interrogate witnesses, suspects and others involved (police specialists etc) collecting (hopefully) information and clues as you go.
People will be able to work individually or in pairs or groups or just tag along for the ride.
Once back at Keble, you will be able to discuss and think about the information you have collected. I'll give a sealed envelope to someone sober and trustworthy (in our crowd??

) so you know I haven't changed my mind mid-stream.
Later in the day (or whenever you say) you can share with the group who you accuse of the crime and we can open the envelope. (It won't be complex, difficult, intellectual or particularly challenging in any way, I assure you.)
Does that sound like something you'd like to do?
If it is not to people's taste, I can always go back to the drawing board.
Arphy