Can someone please help me with baking a erscotch tart? i cant seem to get the filling to set!?

I have used ready made shortcrust pastry and 'baked blind' for the outside, and for the filling i used 175g of light muscovado sugar mixed with a small can of evaporated milk. i whisked them together with a hand blender for about 5mins and then put it into the pastry case. recipe says to leave it in the oven for about 10mins but it hadnt set after then so left it for about half hour and it's still runny! please help. i am trying to make it as a surprise for my boyfriend as it's old school dinners favourite!