I would advise you forget that & go buy some decent trade tile adhesive from a proper trade tile outlet; their powder adhesive is better than the tub mix but you would be ill advised to risk it. You can get BAL Rapidset Flexy cheaper than that even at Topps if you try hard enough or use one of the other reputable trade brands, Webber, Mapei (but not the stuff in B&Q), Ardex etc. You really get what you pay for with tiling products, go “cheapskate” on addy/grout & you may live to regret it. Do a Forum search & wider Google search for more info, it’s well documented; be wise & heed the warning!
More importantly, is this a ply overboard or a replacement floor? What thickness WBP have you used? How have you fixed it? Have you sealed the back & edges? Are these natural slate tiles?
Tiling isn’t rocket science but there are still many things that can catch you out, particularly with a suspended timber floor. I would advise you read the Tiling Sticky & Forum Archive posts before buying materials or going any further or it could prevent you making disastrous & potentially expensive mistakes. It’s important to use only quality trade tilling materials of the correct type for your tiles & tile base; cheapo own brand & DIY stuff is mostly crap.