I'm going to do some blatant plugging & confess that my company distributes the stuff, but that's because it's the best
Get yourself some Leather Therapy water repellant, which coats the leather with an acrylic polymer mesh which waterproofs it but the leather can still breathe.
I won't bore you all with a lecture on the leather tanning process but wax or silicone treatments help waterproof but won't do the leather any favours. I'm at the Llangollen show this weekend with a stand if you are going otherwise a google search should sort you out