Also, you could try emailing scanspeak to ask them where dealers have stock.
Aside from trying eBay/pinkfish etc for the drivers alone, you may be lucky enough to find a pair of suitable “donor” speakers to cannibalise - not only SL2s used them. It might be worth doing if the SL2s are worth enough to you (in every sense of the phrase).
A number of other speakers used the Ferrofluid version of the D2010 (Spendor being just one example), but Naim were one of the few (only?) to use the non-Ferrofluid D2010. Quite apart from that, any cannibalised tweeters will likely be past their best.
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I think the best options would be either NOS of the correct Part No - or do the work needed to fit the new version (hasn’t someone on here done this, already…?).
YMMV… 
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Agreed - cannibalised tweeters are very likely going to be past their best, but I was mindful that we might be talking about making the best of a bad lot here.
There can’t be that many suitable NOS left and the number out there is not going to be getting bigger!
IIRC the new version of that tweeter from scanspeak is not very different from the classic version.
It may be impossible to fit the new ones to your speakers, but if I needed a pair, I would have a chat with falcon and/or Willys Hifi and ask them whether the new tweeters could somehow be adapted to fit your speakers, and whether they might work with the existing crossover.
Thanks for all the suggestions, I spoke with Falcon acoustics and Willys to no avail with neither having stock left or experience of fitting the replacement 852100s – Willys did say it’s a metal bracket/mount mod not cabinet mod. Needed
I’ve also emailed a couple of others supplier and also Scanspeak to ask them where dealers have stock
My systems active so and tweakable in the lows and highs so if I draw a blank I’ll probably give the 852100 route a try – surely I wont be the first, has anyone done this?
How does the Ferrofluid version of the D2010 differ to the one Naim used Richard?
Well, it has ferrofluid inside which will change the way it performs. Apart from that, I’ve never compared so couldn’t say.
As for the revised versions, the issue is that the terminals are no longer on the far rear of the tweeter casing, but now instead they are on the side edge, which fouls the metal tweeter bracket on the SL2s. It would mean drilling out appropriate holes which I can understand most might be reluctant to do. However, it might just be the only way forwards.
Just to be clear, as an SL2 owner myself, if I were in your position I think I would choose the correct tweeters that are currently available and then drill out the arm to suit. This is a much better idea than fitting either old/used or incorrect tweeters.
For reference, the correct version of the new tweeter is the D2010/852100 without Ferrofluid.
I was in the past reluctant to embark on any job which required modification, unscrewing, soldering drilling etc. once you commit it’s not actually that difficult especially if you can find someone else with the skills to help or just learn them and get confident. It really is the only way to get the best outcome. What I am trying to say from experience is don’t dismiss it out of hand. 
I still have a new unused set of SL2 Naim spec tweeters. (Did you know that @jonoB😉)
Had every intention to mount them but never had the need at the end of the day, at the time of purchase a few in the forum were suggesting to change out every 10 years to receive benefits. I’ve since been advised to change out when they don’t work. By the time they landed in my country the cost was insane with customs tax etc. thought about moving them but the money I would ask would make it laugh money.
I feel for you and apologies.
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I think a lot of us here with SL2s bought up spare sets of the old Scanspeak D2010/851100 tweeters as soon as we heard that they were to be discontinued/replaced, myself included. I haven’t changed mine over as I reckon my old ones still sound really good, although I’m tempted to do it just to see what I might be missing…
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I suspect you would be pleasantly surprised by the change.
I changed the tweeters on my NBLs when they would have been about 15yrs old and the improvement was astonishing - I had not realised how “off” they had been.
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I think you would be advised to do some indepth research with the remount of the sl2 tweets. Hot glue gun for the OEM tweeters, not sure if the new replacement tweeter binding posts will be close enough for the wire to reach, hylomar blue mounting sealant, .85 Nm screw torque or finger tight. 505 crystal solder. Sure kinda easy enough to do but if your a green horn watch the heat etc.
Stop it 
No need for Hylomar on the tweeter of the SL2 as no need for any seal. However, hot glue yes, and suitable solder depending on whether pre or post ROHS. As for the hook up wire, there’s a bit of slack there but not sure whether it would reach so possibly better to replace with some suitable RS cable lengths. If it were me, I would solder to the tweeters and then hot glue first before mounting. Then solder to the rear terminals once fitted. The tweeter itself might need mounting slightly offset or set with terminals north/south so the drilled holes can clear the lateral mount plate. An interesting little project for sure, but one that will need to be tackled sooner or later by somebody…
What does the hot glue do Richard? I know where it goes, but why?
I thought you were going to relocate that last pair of SBL tweeters that were on the shelf in the store room……. 
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Shhh…
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Yes, boss!
On the original tweeters the hot glue is applied to rear of the tweeter and covers the soldered end s and terminals. As for why, I think I knew at some point in time but for now cannot recall…