I guess im qualified to answer your question as i didnt do Eco.
I start at Johns in October so cant tell you much about how im finding the course. I didnt do further maths or eco. The interview was surprisingly easy (which made me worry immensely!) It was just some very simple theory about what causes a shifts in a demand curve e.g. changes in consumer taste, income, price changes of complement or substitutes etc. That was it!
I dont know how rare it is, but i know of at least one other on TSR who got in without Eco.
All you need is a reason why you didnt do eco. In my case, it was because my school doesnt offer eco and the nearest school that does is about 20 miles away.
I dont really know if its easy or hard to get in without eco, but for what its worth, i didnt get ripped to pieces in the subject interview.