Well in the end, I went for some Alpines, from their SXE range.
Now I'm not the brightest person, and I got most of the door card off and then stuffed the tape measure up and "roughly measured the door speakers" to be about 6", so I ordered a pair of 6.25"....
Imagine my surprise when, several weeks later I got round to fitting them and discovered that I was a bit wrong, looks like the factory ones are about 5.25" or thereabouts.
so after some pondering what to do, I went ahead and decided to fir the 6.25" ones that I had bought. So some grafting was required....
I removed the existing speaker and the plastic mount, took the plastic clip off the speaker connectors.
held the speaker up into the door, checked the window wouldn't hit it, then found that I could use 2 existing holes with only the need to drill 1 more (3 would be enough).
So drilled hole at the top, then used some of the screws that the speakers came with to put them in, with some tap washers to act as a bit of insulation between the screws and the speaker.
So sounds ok, though I have only done 1 so far.
But I did wonder whether I should swap the tap washer around so it sits beneath the speaker, that would then push the speaker out a bit and away from the door panel
Went to put the door panel back on and part of the top window rubber strip had come away from the panel so I need to re-affix that as the fixings had broken off (someone has been at the door panel before)
so this is what a 6.25" speaker looks like when grafted into a Rover 600 door panel....