Silverside Beef Roast - tough to cut on the plate and chewy
Silverside is one of those cuts that varies a lot, I've had topsides and silversides that came out reasonably tender and juicy, and I've had ones where I need power tools to cut them, even when I do everything right. They're cuts I avoid as a result. 

Silverside is a cut that comes from the rear of the animal, it doesn't have much marbling and does a fair amount of work, so has lots of connective tissue which toughens it. Cattle breed and how the animal was raised make a big difference in whether you'll get a good result. If you do want to buy it then be selective, look across the cuts the supermarket has for marbling. Stick a thumb into them to see which one is most tender, and pick that.

Tweaking your method would help you get a better result. 180C fan is too hot, you will overcook the outside before the middle is done, and you will have less time for connective tissue to break down and make the result tender. I like to start mine hot to get a good crust, but after 10 minutes I would turn it down to 150C at the highest, 140C would be even better. Next you need to rest it for much longer than 20 minutes. When you rest the meat it continues cooking, connective tissue keeps breaking down, and the meat relaxes. This takes time, especially for tougher cuts. Some professional chefs say you should rest it as long as you cook it, I'm not in that camp myself but I'd say wrap it tightly and give it at least an hour before you cut it.