Disclaimer: while this did work, and the insides look as if they're designed for something like this, I'm not sure if it was the exact correct intended approach. Follow these steps at own risk!
Based on Tester101 and Ed Beal's suggestions I tried pushing the ends. I think it was spring-loaded at both ends - by firmly but gently pushing the holder on one side, and firmly but gently holding and pushing the tube itself close to the opposite end in the other hand, I was able to get it loose.
The gap in the upside-down-U shaped rim was then just the right size to allow the (untwisted) pins to come through, which makes me think this was probably (but not definitely) the intended way. There are also what appear to be grooves for the pins.
Both sides were capable of bending backwards about 3mm-4mm with some force carefully applied (it felt like any more than this either side would crack something). This wasn't enough to free the pins on either side, but when the same force was applied to both sides at once, they could come loose on one side. This would be a much easier job with a second person - you could pull both holders apart while the second person holds and manoeuvres the bulb.
Getting the bulb in was slightly trickier, and with hindsight I'd definitely recommend having a second person help. If you do attempt it on your own, set your stool up next to the side where you're pulling the holder, then extend your arm as far as possible down the tube to push the pins in on the far side first, pushing that far side out out using the tube, so you're up close for the tricky task of getting the second pair of pins in the hole without them getting caught on the casing.
Here's what the inside of the fitting looks like with no bulb. It looks a little like a tragic weeping ghost from a Studio Ghibli film. Sad light fitting is sad:
Update: due to national power shortages I couldn't test the new bulb until much later, when I did, the bulb instantly blew. Bright light at each end, then nothing. Don't yet know the cause of the problem, and it's likely it's the same as what blew the old bulb, but I thought it best to mention just in case it's something I did.
I see no reason to include an insulated neutral. (40-years in the business.) One thing you may or may not be aware of when making the splices in the J-box behind the new range, use an over size box. And #2 use something better than a large wire nut to make the hot lead connections. Polaris makes a good splice connector. That kind of current can cause real problems with poor connections. Wire nuts were never designed to handle those kind of loads.
Best Answer
The CE Mark
The CE mark is an unverified promise by the inside-the-EU manufacturer ... or the inside-the-EU importer. It promises that the item complies with EU safety standards.
It is not anything like UL, CSA, TUV, etc. - those are independent testing labs and their staff has actually run a specimen of the product through a battery of tests, everything from overloads to flame-testing (for toxic substances) to sourcing of components.
You're the importer. You're responsible.
Anyway, when you buy via Amazon/ebay/Ali, you are importing this into the EU. So if it has the mark (obviously, it's counterfeit), you are agreeing to test it according to CE standards (and of course it will fail).
If it does not have the CE mark, then it's illegal to install it, unless it has the mark and listing number of a proper testing lab like TUV.
Amazon is a slippery one, because they seem like a domestic mail-order supplier like Wickes or Redoute who you would expect to be reliable. But Amazon intermixes into their listings something called Amazon Marketplace, which is a flea market of third party sellers peddling the cheapest junk in Shenzhen.
People confuse these "Marketplace" flea-market listings with listings actually sold by Amazon proper. The scammers write a bunch of fake reviews*, and often charge similar prices to quality domestic goods, and people buy it because they think the high price means quality product. They've really got it down.
The products Amazon buys for their own internal sale are usually high quality normal goods like you'd get from Wickes.
The wires
If you find a proper UL listing or equivalent, then you can install it. Due to treaties, a CSA listing on a US product is honored in the EU. (again CE isn't anything, but a TUV listing from a German product is honored in the US). So it's not uncommon to find EU color codes (brown/blue) in the US or US color codes (black/white) in the EU.
Black is hot and should be taped brown (not critical).
White is neutral and should be taped light blue.
* They get "verified sale" on their reviews thusly: Initially they direct-ship the items, place hundreds of orders from sockpuppet accounts, don't actually ship the item, collect the payment from themselves (eating Amazon's cut), then write a glowing 5-star review of themselves. Then with their 4.7 star rating, they place it in Amazon Fulfillment so it's drop-shipped from an Amazon warehouse.