Is the wall face over the window fairly level? (ie you won't be packing the tiles out much) can't speak for wickes brand products, but as a rule diy/own brands tend to be cheap for a reason. Get yourself some BAL wall adhesive (the cheapest is the green which should suffice in a non wet area) make sure you get the bed even and tap in with your rubber mallet (they may take a bit of 'twisting' to bed in securely) you may experience a bit of drop as they go off, just keep an eye on them and push in as required. Failing that, pull a line off of the corresponding course to the top of the window and pin a batten across the face of the window to support the top courses, if you haven't set it out as a whole
tile to the window but a cut, just fix the course over the window span..as always level..level and level! so that you don't come off as you carry on around the room!..sometimes if they are particularly heavy
tile this is easier, and you can carry on fixing the other side of the room..like I said tho' LEVEL!..lest you be in serious trouble!
..best of luck mate