Two sets of tools that I would not be without now are two sets of diamond needle files one set was bought from Aldi and the other smaller set from Micro Mark in the US of A.
One thing I would not say was essential but handy is a mains powered Dremel with a flex shaft (Axminster tools), I don't use it for drilling as it's too fast but its ideal for slitting discs (cutting piano wire) and burrs for cleaning castings etc. I tend to use the collet chuck with it not the three jaw chuck.
A face plate is handy for checking the flatness and marking out of things (but a proper one is quit expensive) if you keep an eye out in places like Wikos, Tesco Etc you can get a granite worktop protector for about £10. The best price I've seen is about £6.00 in Home Bargains.
OzzyO.