Thanks for your replies.
Since these are my fish and I will be moving next year (am currently at my parents' house), most likely no more fish will be added, so 1.5m sounds good. I was tossing up between pond liner or a large container from somewhere like payless plastics if they're big enough. I notice though that a lot of containers/pre-formed ponds are black, does this heat the water up too much? Also while trawling through some of the threads people have mentioned their plain pond-liner ponds developing small holes and leaking, is this much of a problem? Thanks for the tip about fencing requirements too, I wouldn't have thought about that! I thought about a bathtub, but don't think my parents would appreciate having a bathtub on their back lawn, no matter how nicely decorated it would be.