A quick update on this:
I've bent the new bracket as Ethel suggested; there are existing bends in the bracket which I used to bend the bracket out then back to parallel where it passes over the original item. After doing so, there was a need to enlarge the holes in the new bracket to ~11mm to allow all 3 bolts to line-up. Be warned, the thickness of the new over old bracket means that the 5/16 UNC which passes through both brackets no longer engages the gearbox as it did with only the original, and I managed to strip the thread seeing as I had put a plain and shakeproof washer. I took off both and managed to grab a couple of threads and did not tighten it much at all. The other bolts all tightened up fine. Also to note is that if you remove all 5 bolts at the same time, then it appears that the engine starts to cry a bit of oil from somewhere in this area...
I have ordered a 5/16 x 1¼" UNC to replace the standard and aptly-named DAM7756, which is 1 1/16" long. This will mean that I've got ~4.7mm (in new money) to play with- ~4mm for the new bracket and 0.7mm which can be taken up with a shakeproof washer.
There is an added complication, in that when the new stabiliser bar was offered up and the site for the hole that is to go in the rear subframe leg was surveyed, it seems that the bar is about 5mm too short to reach a point which would allow a nut to be placed on the opposite side. Looking at the other side, where the L/H MSSK001 kit will go, there is already a hole there where I've read that the L/H stabiliser bolts through.
I think the reason for this is that the kit was designed using dimensions not for a later 1275cc, which has the engine mounted slightly further forward. Why there couldn't be a longer slotted section on the bracket, to cater for all options, is beyond me. I plan to elongate the slot slightly (in-situ using a Dremel and tungsten carbide bit), to see if I can generate the extra reach that way (although the slot ends pretty close to the edge of the bracket), or otherwise I will need to buy an adjustable/correct length stabiliser bar.
At this point I downed tools as my willpower was at rock-bottom.
I will update once the job is complete.