Tensioner Spring
I had the same trouble with the tensioner spring flying off.
If you follow the Haynes manual for putting the timing belt back on, it say something like turn the cam (Upper one) 2 turns anticlockwise with the tensioner loose to allow it to correctly tension the belt. (Use a length of wood with two screws correctly spaced to turn the CAM Shaft).
Unfortunately it seems to fail so mention that YOU SHOULD DO THIS WITH THE SPARK PLUGS REMOVED, otherwise the compression in the cylinders causes the Crank Shaft (Lower one) to jump round and the tensioner spring flies off!! If your not careful the belt may slip too!!!
If the Belt does slip, then slide it off. Rotate the crankshaft whichever direction takes it the shortest distance so the TDC Marker is at either 3 o’clock or 9 o’clock. Now the pistons are all in a line in the centre of the cylinders (no chance of valves hitting pistons). Line up the CAM Timing Marks, then the Crank Timing mark (Shortest distance i.e quarter turn!) Belt on and try again.
I'm sure ED can confirm this, as he is the master to be able to do this in an hour, takes me just over two!!