Thanks for all the comprehensive replies! When I drained it the element looked a little dirty so I gave it a clean and I thought I'd give it another go. It worked, great! Just about to put the hops in and it cuts out again!! It was overheating as you could smell the element when the wort was drained.
I took the element out for a closer look. Some parts scaled over underneath but the coating is coming off revealing the copper underneath. I'm hoping I haven't just wasted a day, and a load of grain. Grrrrrrrrrrr!
My advice to anyone building a boiler is to put 2 elements in, its what I'm going to do this week!