The point, Confused CH and JBM, is that there may not be a legal answer as clear cut as you would want. You can look at the contract of employment and see what it says. You can look at the job description, if there is one, and see what it says. Probably if the drafter had any sense it will say something on the lines of "such other tasks as may from time to time be necessary for effective working" or similar.
Then there is the situation if everybody else says OK, we want to keep the firm going and you are the one who says no, what will the work situation with the rest of the staff be like?
The boss may take a "legal" line and say" I am giving you notice that this is now included in your job". Then it is up to you take it, or leave and claim constructive dismissal. Then it will be a matter of whether that change is considered reasonable or not, so there is no absolute definitive on this. It will be down to a tribunal to decide on the facts of the case. I think the "preaching" was just to give you the feel that you may not be in the best position to stand on your high horse.