So my tank-top ended up as a cushion. I felted it first by slinging it into the washing machine with a pair of my jeans on a 40 degree cycle but this does only work if your knitwear is labelled as hand-wash only (machine wash wool is treated not to shrink or felt) and made up of 100% wool.
This shrinks it down nicely and felts the wool so it can be cut without fraying and I used an old cushion pad.
I have felted quite a few jumpers in the past in this way and I am now working on my PhD (Projects Half Done) in 'Sewing up Old Felted Jumpers into a Throw'. Well these things take time!