I can't answer the OP's query but have an anecdote re the Merchantman & speed. I was on Approach Radar at Glasgow in the late 70's when I had to ask a Merchantman to reduce speed from 250 knots as he was rapidly hoovering up the Dan Air Comet ahead of him. I'm sure there was a chuckle in the ABC Captain's reply when he asked what speed I would like.
