I will contact her again and try get her to write it down as I spoke over the phone with her, a lengthy 2 hour conversation .
1) This exemption doesn't state commercial BP it just states BP in general and to what I was told also meant home made black powder made via a person who holds a COER certificate and is less than or equal to 100g. The BP can be stored if you have an acquire and keep COER.
2) This exemption doesn't state CAT 4 fireworks but then neither does it state any hazard of fireworks so could it apply to any hazard type? this is the problem who knows the law doesn't purely specify it and this is what she said when I asked her. So I assume you can also assemble, disassemble and fuse CAT 1, 2 and 3 also because it doesn't really state otherwise. It may say for display purposes but then again you can make displays with CAT 3 also.
3) The explosive inspectorate told me I could make any composition as long as its NET weight didn't exceed 100g and with clarification from some of her other colleagues stated these could be put into devices as long as the total devices NET weight didn't go over 100g in the same building. Again the law doesn't state that you cannot make devices from these experimental compositions but just states you can make 100g of explosives. So since it doesn't specify, what exactly can we and can we not make? again who knows. Also regulation 9, paragraph 2, sub-paragraph. (a) basically says we need a COER certificate to manufacture, acquire and store the composition if it is part of the Control of Explosives Regulations, "but nothing in this sub-paragraph shall be taken as authorising any acquisition or keeping of explosives for which an explosives certificate is required by virtue of regulation 7 of those Regulations, without such a certificate;".
On site mixing licenses are useless for us pyros as it only applies to ammonium nitrate mixing for on the day use. I do not see anywhere were it states we CANNOT manufacture fireworks under 100g but then one may argue it doesn't say you can either but then this is were the solution to that argument lies... no one knows. I am by all means not an expert on the MSER but have read over it many times and also got a printed copy. What I have said above is gathered from my experience in reading MSER and from those who I spoke to from HSE, Fire Service and police.
1) The problem is that the BP that you make is not BP as it does not have a UN classification number allocated by an authorised manufacturer so you are still stuck with the 100gram limit and can't keep making 100gram batches and storing them up to the 10kg. This 10kg rule only applies to BP and shooters powders etc, so even if it did apply it would somewhat limit the effects that you could produce as it does not cover the storage of loose powders for example colour compositions.
2)Yes it does not state cat4 in the regs, but it is implied as the modification of any consumer firework be it cat 1, 2, 3 or 4 automatically renders it a cat 4 device which of course is illegal to own without the right documentation.
3)Yes I agree the 100gram rule applies for experimental purposes. But as stated this 100gram total at any one time so no storage above the 100gram limit is allowed without a manufacturing licence (read the ability to assign the appropriate hazard category and store appropriately in accordance with the given licence). Yes it could be argued that the regulations do not prohibit the manufacture of an experimental device (I have heard on the grape vine from other UKPS members that the HSE frown upon this), but if it is a 100gram flash device i think you would be on very shaky ground as it could be deemed to be an explosive deemed for malicious use which instantly put you into the terrorism laws which don't care about 100gram limits.
If you don't have it fork out the £20 for the ACOP (approved code of practice) to go with the regulations it can be bought from the HSE website ISBN no 0-7176-2816-7. Not exactly a riveting read, but it certainly helps to explain the regs especially if you intend to get legal.