Having stripped my ignition lock in order to try and remove the barrel (and failed), i am having trouble getting it back together. It is a struggle to get the plastic components (with their springs, copper plate, ball bearing) all in place - when i do, it doesn't feel sturdy, and the whole mechanism doesn't feel solid enough that it would last. (Not sure if there is a trick to fixing the copper plate down over the springs before refitting the plastic top)

I would be OK to buy a complete replacement, but I can't track down a complete Series 3 ignition lock (petrol).

Is it possible to get something to replace just the plastic components (ideally something sealed that isn't a struggle to rebuild) to fit on the existing metal parts?
Is is there a compatible replacement (e.g. from a Defender) that can be used instead?

Not sure what year your S3 is, but if you can find some sort of parts compatibility list or even the BL part number to cross reference then I'm fairly sure they must have have used this on other BL products. Can't imagine they designed an ignition lock for just a single model and it might help to track one down if you widen the search to other vehicles. For example, I have just had a look at the key for my 77 MGB and it's almost identical to the one for my 82 S3, also put it against the key for a mates 90 that's with me for storage and basic shape and dimensions are identical to the S3 one so might be the route to follow. Cheers......Ian

The 90 one fits and is still cheap and available. I’d save the switch bit on the back of your original just to make sure the connections work. It does fit on the new one.
I had to butcher a modern petrol one for a series diesel with the pull stop. Managed it but it’s a sod if a job. The petrol will be much easier.