Both finish in a day or two, so thought I'd flag them for you. Of course the price for Lost & Damned will probably go up towards the end of the auction, but certainly in the past I've seen plenty of copies of Slaves to Darkness go for more like £60-70 in the past. Slaves to Darkness was also republished by GW, but only available at Warhammer World. I've seen some of the members on here offer to pick up copies for other people in the past, but I don't know if it's still stocked or if it was just a limited run GW did. Best of luck in your hunt, both very good books.
I threw my originals away in the early 90s but I just bought 400 painted demons this week so I guess I need the books again, unless I am playing firstved where are hey are summoned and generic