Thanks for the comments guys - I do have a couple of longer exposed pics with the headlights streaking by which I'll post when I get home. Unfortunately I didn't have a tripod with me, and my hand was ever so slightly unsteady at that hour of the morning (and after several café cognacs in the bar), so they're not quite as sharp.
How did I get there? Easy really. "There" is the tower in the Welcome enclosure. How is by buying a ticket to the spectators enclosure for 19 Euros.
We couldn't get in to the spectators enclosure before 6pm having bought the ticket trackside, but after that it was fine. Before 6 we could still get to the bar, and nice clean toilets though - that part of the Welcome has become public. They also seemed to be the only place in the circuit still selling cigarettes (under the counter), which was a blessed relief!
The viewpoint from ground level was ok - above the fence too, so I got a couple of 'clean' pics of the cars from there. Seemed you could go on to the top of the tower pretty much any time apart from the last hour or so of the race (and it was re-opened again at 4:01), and to top it all off, they opened the gates to the circuit once the cars were in parc fermé so we didn't have to climb over anything to get onto the track

Highly recommended, even if you can't get right up next to the pit-lane exit like you could in the 80s...