To add to Northerner's post, we in the tower look out for the ident as an indication that the aircraft has two-way with London.....absence of ident prompts us to try again to see if the a/c is still on our frequency.
To John R81, in the scenario you describe, what would you expect the ATCO to do if there was an 'aircraft' (A320) just slowing down after landing and vacating, but clearly within your field of view, and another 'aircraft' (PA28) on short final?
If I was that ATCO and you read back 'aircraft' I'd expect the worst and assume that you meant after the one vacating. While my heart rate shoots up I'd be trying to get on the R/T and confirm.
If you don't understand the clearance you've been given, you must say so. If you don't know the difference between an A320 and a PA28 then you must say so, and the ATCO will come back with clarifying remarks. By just reading back 'aircraft' you are already passing through the first slice of cheese.