As a further update...
The bottle of CataClean was bucked into the (quarter full) tank last night, and I took the Laguna out on a fairly brisk voyage along the entire length of the M5 and M3 from end to end (though, being Northern Ireland, I should point out that these fine pieces of road only cover less than five miles put together), and then onwards to Bangor - so about half an hour's running in total, at between 40 and 70-ish with a few stops for lights and roundabouts.
As I said earlier, it may be my imagination but she seems to be running significantly better - much more willing and sprightly than before. When I bought the car, the rear bearings were grumbling away ominously and although everything sounded much happier after I got them changed, it still seemed just a smidge sluggish. Obviously the dodgy coil pack was the main problem, but is it possible that the new ones are feeding all the plugs a bit better? I don't really know whether static units like this do start to lose efficiency over time, so maybe the replacements have added a little bit of pep to the proceedings. If they were indeed the original units, as I suspect from the lack of burring to the Torx retaining bolts, then they're close on 20 years old.
Changing the plugs may also have made a difference - if the spark gap is critical on these 8v units, then the electrodes on the old Champions had burned away 0.2mm below (what I believe to be) their pre-set 0.9mm gap, which may well be enough to affect the running. The replacement plugs aren't new, but are way less manky than before.
I may chance my arm online for a brand new set, as no stores seem to stock the HBOL prescribed plugs (Bosch WR8D+) or those recommended by Renault (Champion N7YCX). It's currently rocking a used set of NGK BPR6Es from the dormant silver Lag, and seems pretty perky with them in - so I'm tempted to just get a new set of these, since they seem to suit it. Any thoughts? I messed about with Bosch Super4s years ago, and although I liked to think they made a difference (in a 1256cc Viva), I'm now not so convinced... but if anyone has any suggestions, I'm all ears.
Also looking to fit a new set of HT leads, while retaining the old ones as spares. Although they seem to be working okay, heat and oil have deformed the rubber flanges around the plug connector ends, and I figure that I'd rather just replace as much of the ignition system as I can to hopefully fend off future problems. If they're old, as I suspect they are, then they may be breaking down a little inside too.
What I'm most surprised at is a few other unrelated things that appear to have spontaneously fixed themselves.
As bought, the digital odometer was blank. The seller told me it was nothing to worry about, it just needed a pair of dash bulbs slotted in. After a fun* morning dismantling the dashboard (yeah - HBOL describes it as 'simple' and shows the instrument cluster just lifting straight out - but makes the charming assumption that you've already removed the steering wheel and airbag assembly just for the hell of it, like in their smudgy photos. Rather more of a faff with the wheel still in situ) I replaced the bulbs and screwed it back together. Odo was now appearing fine, though one half of the cluster then began to go dark intermittently. A sharp smack to the side of the binnacle sometimes brought them all back on, but obviously this isn't good for the PCB as a long-term solution. Curiously, since changing the coil packs on Tuesday night, the whole cluster has lit up properly each time I start it, and stays on. Hmm.
The car also had the original cassette player fitted when acquired. After a few weeks of reacquainting myself with some old 90s mix tapes (Cast, Northern Uproar and Catatonia, anyone?), the rubbish muzzy sound quality and tendency to flip sides mid-song annoyed me, so I pulled it and slung a cheap aftermarket Pioneer MP3 head unit in. Now, I had all the aftermarket stereo leads from a Blaupunkt CD player currently living in the silver RN, including steering column control interface leads, so thought this would be a simple swap with a only a change of patch lead needed from Blaupunkt (pin block) to Pioneer (3.5mm jack) - only it turned out to have a slightly different Philips wiring block for the control interface, meaning that I had to buy a whole new set of connectors (costing more than the bastarding head unit). Well, it was working anyway, and I could live with the blank space next to the digital clock where it once used to display the cassette direction or radio station.
The blank space now states 'RENAULT'. Which is kinda cool, but also makes me think - why? Why now? It's not like I even disconnected the battery like the HBOL advised while bolting in the new coil packs.
And yes, I know this makes no sense whatsoever, but the slightly notchy second gear is no longer notchy. I have no explanation why that might be. Usually it needs a little extra squeeze to slot it in fully, but not yesterday. Not this morning either. And the clutch - which is a bit worn and has a tendency to snatch - feels a bit tighter. I'm hoping that it's the Gods of Shite smiling upon me for persevering with this particular heap rather than just ringing the metal recycler, and not a warning sign that it's about to shit itself big time.
But yeah. So far, improvements. Let's just hope they last.
Oh yeah, maybe I should point out at this juncture that I'm planning on driving this car from the Belfast area over to Shetland in the spring, so I'd quite like it to be as reliable as possible.