There is nothing magic about Proofide. Any real leather conditioner will work. I have used Lexol from an autoparts store for years because it breaks in the saddle immediately. Others use neatsfoot oil, motor oil (really!), and snowseal. In fact, Proofide is harder to use because it does not come in liquid form and has to applied with the saddle and the can of Proofide left out in the summer sun to warm up. That is why the Brits always thought it took years to break in a Brooks saddle, they had no summer sun....
Any idea how to tell if the saddle needs retensioning ?
My experience with Brooks saddles is that it's probably a good idea to be very cautious about tightening them. I've got two B-17's and a Team Pro, and I've never tensioned any of them. I think if there's no obvious problem with the leather being noticeably loose or sagging, just leave it alone. I remember reading on Sheldon's site that an easy way to ruin a Brooks is to tighten it unnecessarily-
I have my own blend of the original proofide ingredients: beeswax, tallow, citronella oil, vegetable oil (olive), paraffin (candle) wax and cod oil (smells like fish, therefore citronella is needed to get rid of the smell)
This home brew stuff works fine. It is much cheaper compared to the ridiculous high price of the original proofide (which is about 10 $ a can).
I guess any good quality product designed for maintaining a horse saddle would be ok for caring about your brooks.
I've treated the underside with nikwax and the top with renapur.
I still suspect it needs tightening - is there any definitive way to tell if the tension is too low ?
Ride it and see if it "holds you up" the way you would like. If not, give the screw a half-turn and try again. I'd assume the idea is to maintain the resilient feel as new without straining the leather. That said, I haven't really had one sag significantly ever.