@epicurus, sorry I misread your original offer post. you're totally right, it's a refund IF LOSE, so if your 1st Unibet bet wins - then you're all done.
Let's look at a few examples:
C.Palace vs Everton - this example assumes 0% exchange commission:
UNIBETBack £40 on Crystal Palace (odds 4/1...
BACKING: Treat the first Unibet £40 bet as a risk free bet. Look for reasonably high odds, say 4.50+.
LAYING: Treat your lay bet as if Stake Not Returned. For 4.50/4.50 back/lay odds, lay around £30.
a) If 1st Unibet bet wins, your +£140 at Unibet minus £105 lay loss equals £35 to play with...