Bit confused?!

Amazingly, after I've measured it, the bottom bracket width is 68mm. Which I guess means it's 24tpi
? Felt certain it'd be 71mm/26tpi with it being a 1982 lower end Carlton.

Inner ring chainline is 41.5mm, can I get away with running it 1.5mm out from the 43mm at the back ? I'll probably end up with a new bottom bracket at some point in the future though now it looks as though it's 24tpi

1.5mm will be fine.

Lucky for you it's out the right way in the sense that if you want to 'straighten' your chainline at a future date, you can buy a wider BB without any risk of the chainring fouling the chainstay. 3mm wider BB should give you 1.5 chainline movement.

Also, shell width is no guarantee of thread pitch. Checking is best policy. I'm lucky enough to have a thread gauge that has 24 and 26tpi gauges, measuring by eye and counting threads makes my eyes go funny! However, I find the best way to count as deSelby says, measure out half an inch on the threads and mark the ends of the half inch with a pencil - in the grooves, starting one or two in from the outer face of the shell (end ones are sometimes a bit vague where they are worn) count the grooves with a pin; the first groove you marked is number 1, don't count the groove containing the other pencil mark or you will have counted too many.

@deSelby: I was writing that stuff about Carlton from memory, couldn't remember which site made the high end models. Thanks for the correction.