This ^^^ These are what I have for the Spitfire for the bigger flanges. And yes they also have the collapsible spacers.

Yrp! I think it's his thing, judging by the Commer camper...

You may well be right that all Spitfire 3.63s have collapsible spacers, but I think some 3.89s do as well! Sounds like it’s got to come apart anyway…..

Seems like an odd choice. From what I’ve read, although simple and tough, they are definitely not built for speed and road manners are “interesting”!

Dunno if it’s the camera position foreshortening….. but that looks like a ridiculously fat beam for its length! (Colin did remark earlier on the building regs lot liking fat beams) This unfortunately. I’m sure they won’t be offering to contribute to the work made necessary by their utter failure! Yes, as they all are . Have had similar in the past with Western Power, though that turned out in my favour as their previous work was so very shoddy and affected a neighbour as well, so they ended up putting it right and taking most of the cost of the modification I needed in the process!

One semi-certain way to tell, if it has a castle nut on the pinion, which denotes a solid preload spacer it will be a 3.89. I think all 3.63s had a collapsible preload spacer, which had a nyloc. Of course modifications are possible but not very likely. There are also some odd ball configurations (4.55 or 4.11 from Marina or Dolomites), but 3.89 will be by far the most likely option for a round flange Spitfire diff with a castle nut.

Progress has been slow this week as time to spend on the car has been limited. Not helped that I picked up a diff from ebay last night.  A bit of a gamble as it has an unknown history other than being sat in a garage for 20 years.  I think it's the correct original unit for my car and the flanges match with the unit currently installed.  At the very least I should be able to clean, paint, drill a drain hole, then rebuild my diff into this case with new seals and hopefully no leaks. A quick search online suggests it's a type D marked and with FH so from a Mk4 spit, and so better than 1500 i have fitted which is a bit long for me.  If this is the factory internals then it should be a 3.89 ratio which will be slightly more peppy than the 3.63 I currently have and I think it should correct the speedo readout.  One issue is that I can't confirm the ratio as the driveshaft input is seized as least as far as turning by hand.  I think I'll have to take it apart whatever happens so I might just do that and see what's what.  Definitely a job for after the car is back running again!

Extra places for festive lights! I'm entirely with you on not involving the powers that be. It's not fallen down yet, you'll make it better than it was, and you have the piece of paper saying it's fine.  The only advantages I can think of in engaging with them would be either to acquire evidence with which to sue the original builders (probably not worth the bother, assuming the builders even still exist), or if you were looking for the correct way to repair it.  

Thanks for the vote of confidence. I was kind of hoping this was the one part of the works that the previous occupants had carried out that wasn't shoddy. Not sure why I thought that, only the building regs side of things reassured me. Council offered to look at some photos of it and decide if it needs reassessing, but I don't think it's a good idea to take them up on their offer, nothing good can come of it, only more expense for me when they decide it's non-compliant. I'm not entirely sure what the best way to sort it out is, so may get someone in to do it. I don't like the idea of loading the relatively thin and non-reinforced slab with structural loads if a pier was constructed. I suspect the best option is going to be to simply rebuild both sections of wall in higher density concrete blocks topped with a padstone or engineering brick. Here is a shot from further back for more context: I'm probably going to end up leaving it alone now until the new year, as I'm sidetracked with the gas meter relocation, most of the ground works are done now, but I still need to excavate a little more and add a bedding layer of sand as there is a bit of flint in our soil. I also need to deal with the soil pipe that will become defunct and is in the way. The GDN want 0.5m depth minimum, and 0.7m to allow for a bend at the meter end. Seems to be a "do as I say not as I do" company, since this is their work, they renewed the gas line by inserting an MDPE pipe inside the existing steel pipe,  the tail they've added to the meter is scarily close the surface, lucky I didn't cut much closer to the meter...

There's a chappy on YouTube who was putting a Saab turbo engine in one of these (he already has one in a Scimitar and a Commer camper van!). I don't think he posts videos anymore, but he still does the projects (too much time spent editing, not enough time spent actually doing the work).