The GR4 was in the visual circuit. That means that they were protecting themselves from other aircraft purely by looking out, and that, presumably is what they were doing. The aircraft that they came into conflict with was on an instrument approach. It was the job of the crew in the visual pattern to avoid the crew flying on instruments (and therefore not looking out)
The problem was that the weather was not really fit for the visual circuit, so that crew could not see the aircraft on the instrument approach.
Why an aircraft was in the visual circuit when the weather was not fit is the crux of this airprox.
Neither crew would have been using the radar in an anti collision mode as they both had more important things to do at that stage.