The problem with dosing for carby meals is you really need to know your insulin (insulin to carbs) ratio, which can vary dramatically from person to person. (eg 1 person might need 1 unit for 10g, another 1unit for 1g , and they make 3x and 5x strength insulins specifically for people with high insulin resistance.)
Forum rules mean I can't give dosing advice but 8 units for 210g sounds quite a low amount of insulin to me . (A lot of T1s get started on 1 unit to 10g.)
This is complicated by the fact that you are supposedly T2 and may still make some of your own insulin, but I suspect that you probably just massively underdosed for that meal.
I happily give correction doses on the (not as rare as I'd like) occasions when I underdose for a meal. One of my issues is that once I go over 10 or so mmol/L I become more insulin resistant so I need more insulin to get myself back in range.
The issue here is that I genuinely don't know what your correction ratio (insulin per mmol/L) should be and you obviously don't want to overdose and go hypo. I would suggest ringing 111 for advice but if you can't do this, this is what Dr Google gave me as the advice in New Zealand for correction doses for T2s
Bear in mind that short acting insulin doesn't act immediately (specially when your bg is high, in my experience) so you don't want to stack it (give too many doses when there is still insulin in your system.) I'm not sure what kind of short acting insulin you are using but my humalog lasts for 4 or 5 hours.
1: how do I know if my rapid insulin is working and not “off”. It looks clear, no stringy bits and was taken out of the fridge on the 15th dec and kept in a Frio case since and not subjected to any temperature fluctuations. It hasn’t been used for over a week, does this matter?
In theory it shouldn't be off as insulin is supposed to last up to a month at normal temperatures out of the fridge, but I suppose there is always the possibility that it's been mistreated somewhere along the line
2: should I take a corrective dose when food isn’t involved ie before bedtime? Or should I go for a walk and see if that works? This is the first time it’s gone this high and stayed high.
Personally I wouldn't enjoy a walk very much if I was that high. Are you able to test for ketones at all? As for the correction dose, you really need medical advice for the amount, and forum rules mean we can't give dosing advice.
Do please let us know how you get on and I agree with
@Melgar that in your position I'd want the tests to know if your are still T2 or actually misdiagnosed T1.
And I'd just like to add I'm very impressed with how well you've managed your insulin and blood sugar up to now by low carbing.