Thank you Steve, it's just something i've got used to looking for over the years.
I learned many years ago (when i was doing my apprenticeship) that the stupidest question is the one that isn't asked! I'm sure you can run through all possible scenarios and see how it works out.
A ball park figure way back when CDs were first being made was a 700MB disc would hold an album. That still holds true so a 16GB stick will hold 20-24 albums roughly. Chances are your head unit is capable of handling 32GB sticks or maybe even more. I have a 128GB card in my phone and it's nowhere near 10% used yet and i have about a dozen albums ripped and copied from CDs onto my phone. Normally i use a music player app and stream it and on that app is a random play function. I normally just use that and skip any i don't want to listen to at the time with the button on the front of the head unit or SWC on the Rover.
I still have the £19.99 head unit which i plan on wiring up in the shed. I also have a BT transmitter that is USB powered (i had these before switching to a BT head unit) and basically you can stream music from the phone via BlueTooth and it then transmits on a chosen FM frequency to the radio so i can take my music into the shed or in either car.
I've mentioned the SWC a couple of times now and i suspect people were thinking it's not relevant to the thread. Well as it happens, it is as i also have a second set of SWC buttons (from a Land Rover but they're the same electrically) that i plan on mounting on the steering column shroud on the Volvo then using this to link it to the head unit in the Volvo :
http://autoleadscp2.com/
It's the same as i used in the Rover so i know will work, it's just the aesthetics and ergonomics i'm working on at the moment!
With a later sterring wheel with integrated buttons and the correct rotary coupler on the steering column i could fully integrate the controls onto the steering wheel but i have another steering wheel coming from a later 960 without buttons. That way i don't need to worry about retrofitting a coupler and the reliability problems associated with them and it won't matter if i later decide i don't like the later wheel.
As for using a gas powered soldering iron, as long as you're careful accidents don't happen. It only heats up to ~200C so is hot enough to melt solder into a pool but not most plastics. It will damage and deform them though so i replace the iron inside the reel of solder which forms a nice base for it when not in use and can be placed out of harms way on the floor.
You would have used a blowtorch or similar for your plumbing so i can see the trepidation but a small, gas-powered iron is a different kettle of fish.
https://www.ebay.co.uk/itm/391628372085
That gives you the idea, there are cheaper options around.
With your tank seal, i'd be inclined to check your local Volvo dealer (perhaps Brookhouse Volvo as they're near Ipswich) to see if they have a tank seal, if so wait until the due date for the Lithuanian one then get one from the dealer locally and put in a dispute for non-delivery and get your money back for the Lithuanian one.
If the postie subsequently delivers it then it's yours legally as it didn't arrive on time so has now been refunded/cancelled and the one that has arrived is technically unsolicited.
By law anything sent to you unsolicited is yours to do with what you will, that's how companies get away with sending things out as a "free gift" - a few years back i was sent a smart socket that i've never used but that was sent unsolicited. I'd not ordered it (or even heard of them at the time!) but when i looked into it, a lot of companies were doing that, sometimes to get good reviews on the likes of Amazon, ebay etc, other times just as a "loss-leader" to get their name into peoples home and the subsequent advertising that would bring and presumably other reasons.