May I please just make a slightly sideways reference to Aviation Law in the UK, as opposed to MATS Part 1 which is an intepretation document and not actually a legal document?
The ANO 2005, as subsequently amended by Stat Instruments blah blah, Part 9 does not mention any responsibility for air traffic controllers in relation to separation. ICAO Annex 11, last time I looked, had 57 mentions of responsibility for ATC, none of which were related to safety.
The Rules of the Air Regulations 1996 makes specific reference to the pilots having to maintain safe separation from each other's aircraft.
So, under the laws of the land, if you are on approach to Heathrow in Class A airspace, with full ATC service, IFR in IMC (and add a bit of night in for the hell of it), it would appear that the pilots have full responsibility to give way to the aircraft from their left.
Back to reality, all UK ATCOs should check their Unit Safety Case for the interpretation of the phraseology which has been approved by the management for use at their local unit. It is not acceptable in England and Wales to just take the MATS Part 1 standard and apply it. The unit has to show that the standard is relevant to their operations and that a risk assessment has been carried out.
Interestingly, the laws in Scotland and Northern Ireland are slightly different. However, it would appear that Scottish law is just about the same interpretation as England and Wales now. As for Norn Ieron, well, let's hope nobody has cause to find out! (The CAA may not require you to do a risk assessment for legacy procedures for their purposes of licensing a unit, unfortunately, public expectation is such that you are required to do it, and it is 12 persons from the Clapham Omnibus that will be judging you. "We changed the procedure after the accident..." is not good enough any more.)
And before you fry me, (i) I defend controllers in court, (ii) I spend a lot of time lecturing on the finer points of corporate manslaughter exposure in aviation.
Happy landings and PM if you need to know more.
Discount