Thanks everyone for your advide. I used shims to knock out the element, but it started to strip the copper from the element. In the end, seeing as I was going to have to replace the element anyway I took it off with a hacksaw. The limescale in the boiler is so heavy it completely covered the aperture, and is like bedrock in the bottom of the boiler.
Probably a big "no" but I carefully drilled out a load of the limescale from the end, removed the elements and then started to strip some of the limescale away by hand. I had no idea it could be so tough and grow so thick. So now I'm just going to leave it soaking in citric acid and wait!
Equipment doesn't work? Troubleshooting? If you're handy, members can help.