Thanks for all of the prompt replies, it's appreciated!
We've now spent the evening rearranging our living room and have found space (if we need it) for a rectangular tank. Are there any advantages/disadvantages to having a rectangular tank versus a corner tank?
Also is there anything to beware of if we go for a second hand tank?
Can't say what fish we're thinking of getting yet - is it vitally important to decide if you're having say an Amazonian or Lake Malawi set up? Or can you do a general set up with fish you like??
Thanks again for all the help for these novices!