Ian,
Of course, the answer is, "it depends."
My expertise is in the US system, so I can't speak for the UK, Irish, or any other minimums. However, in general terms it is a question of whether one or both of the aircraft are RADAR identified, what type of RADAR is in use and whether it is mosaic or not, how far the aircraft are from the antenna, and other factors (including wake turbulence and a question as to whether the aircraft are being separated based on RADAR or rather, visually)...
Here's an example of 2.5 mile separation from the US (JO7110.65S,
5-5-4. MINIMA...):
[snip]
g. TERMINAL. 2.5 nautical miles (NM) separation is authorized between aircraft established on the final approach course within 10 NM of the landing runway when operating in single sensor slant range mode and aircraft remains within 40 miles of the antenna and: 1. The leading aircraft's weight class is the same or less than the trailing aircraft;
2. Heavy aircraft and the Boeing 757 are permitted to participate in the separation reduction as the trailing aircraft only;
3. An average runway occupancy time of 50 seconds or less is documented;
4. CTRDs are operational and used for quick glance references;
5. Turnoff points are visible from the control tower.
Based on this example alone, you can see that there are a number of factors in play. Hopefully someone more local to you will chime in as well...
Dave
PS Also note: you said "...compared to legal minima of 1000ft and 5nm..." It's actually an
OR. i.e. 1000 feet
OR 5 nm...
PPS ICAO Annex 11 is also your friend here...