Thank you to everyone who has replied - it is greatly appreciated (and, I am glad to say, very re-assuring). I note a few people say that this is not really like a river cruise - that is fine as it is really Venice we want to see and, as we have never been on a river cruise before, we probably will not be aware of the difference!
Any more tips on Venice and making the most of Uniworld would be most welcome. However, thanks again for your comments which were really useful.