Morph:
I Apologise if I am being dumb (I await the inevitable confirmation of the fact)
Hi Morph,
The only confirmations you’ll get from me are that you’re NOT dumb, and… you were trying to do a job (your boss’ job) which is higher than your paygrade if you’re an employed driver. 
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Caveat:
I’ll start by saying that I believe every word you wrote in your OP. I’ll now add that I’m assuming that the consignor (=sender) has followed their legal responsibilities as set out in ADR.
Morph:
I had one small parcel (smaller than Shoebox, delivery note does not show volume or quantity) on my truck yesterday that was fully stickered up a Class 3 Flamable.
Would it come under domestic Regulations or limited Quantity ?
The fact that the box had a Class 3 label totally removes any possibility that it counts as Limited Quantity (LQ,) because an LQ label is black and white, whereas a Class 3 label is red.
A Class 3 label:
An LQ label:
Surely, even Mr Magoo couldn’t get these mixed up!! 
Still with my caveat in mind… Class 3 substances (not packaged as LQs) will be in one of five ADR Transport Categories (TCs are numbered 0 - 4) for the purpose of giving us a partial exemption if we’re carrying an amount equal to or less that the relevant limit for that category.
TC 0 has no allowance
TC 1 has a 20Kg/L allowance
TC 2 has a 333Kg/L allowance
TC 3 has a 1,000Kg/L allowance
TC 4 has an unlimited allowance…
… as mentioned by Stephenjp
If the ‘stuff’ you were carrying was in ADR TC 0 (which is extremely unlikely,) that would be the only time you’d have needed to grab all of your kit as you did, but then you’d also have needed your ADR Card and orange plates too.
More likely… you were carrying the ‘stuff’ in an amount equal to or less than the relevant ADR TC limit, so all you actually needed was 1 X 2Kg ADR compliant dry powder fire extinguisher and have had some documented ADR ‘awareness’ training. When you’re carrying ‘stuff’ according to the TC limits, NOTHING ELSE APPLIES.
If you’re carrying ‘stuff’ in an amount which exceeds the relevant ADR TC limit, it’s a fully regulated load and all rules apply, such as the need for all kit, 2 extinguishers, orange plates and your ADR Card being on board the vehicle (amongst other things.)
Morph:
I have ADR anyway & grabbed my spill Kit & instructions in writing clipboard ETC. but no Orange plaquards on the truck.
With hindsight, we can now say that you had something of an over-reaction, and that you should have asked your boss.
Morph:
Did I Cluck up?
No, and that’s because it’s your boss’ job to have sorted this for you and given you correct instructions for the work he was asking you to do.
If your boss had had the legally required ADR ‘awareness’ training suitable to his position and responsibilities, he’d have been able to give you a good answer there and then on the day.
Given that your company carries dangerous goods, your boss should seek advice from a properly qualified DGSA to ascertain what needs to be done in order that the situation in which you found yourself doesn’t occur again.
The transport chain comes into play here… It is the consignor’s inescapable legal obligation to identify, classify, fill, close, document and label a consignment of dangerous goods, then communicate that information in a traceable form to the carrier (your boss) so that a decision can be made (by the carrier) as to which ADR carriage requirements apply to a particular job.
Maybe the consignment should have been classed as LQs, but without the missing info on the amount and exact method of packaging, then Owen Money’s observation applies.
It is NOT a part of an employed driver’s duties to act as a surrogate DGSA!! 
I could give you a better answer if the info mentioned by Wheel Nut above were available.