Regarding the mixing/blending of the capers and anchovies and pitted olives, you may be able to do it using a pessle and mortar or a fork and a small china bowl, as long as the ingredients are totally totally blended and put as little of the oil from the anchovies in as possible but don't waste the oil by throwing it.
Then scoop out some of the potato and carrot mixture with a spoon and put the anchovy mixture in carefully, then recover with the potato mixture. As per photos.
Put into centre of oven on high shelf gasmark maybe between 2 and 3 or 200 degrees electric for 35 minutes approximately. Before you mash th potatoes and carrots, why not have a bowl of them to nibble at (as per photo), so delicious. Whether cool or hot, the gentle taste combinations can hold their own with either some clementine segments and is totally yummy even with a gravy (but not too much).
But what do we now use the oil from the anchovies for you might be asking?