Thanks for your suggestion, it really helped out.
So, I managed to get the old bolt out and saved the existing thread at the same time. Got hold of the right angled drill bit adaptor you mentioned and this provided the access I needed. Got in there with a 4mm drill bit as a pilot hole and tried my screw/bolt extractor. Unfortunately, this did not work, every time I've tried to use screw/bolt extractors, they've always failed. This left me with the option of drilling the bolt out entirely and re-tapping the hole as you mentioned. Got the 7mm drill bit out and went until the right angled drill bit was too hot to touch and then the old bolt just fell apart, gave it a touch up with an 8mm tap to clean the threads and a new bolt fit in there no problem.
Got the ARB attached with some new bushes and drop links and now the car feels much better to drive.
Photo below of the aftermath of drilling, excuse the weld mark there, tried welding a bolt onto the old one to try and undo it that way, lack of space caused me to miss and give up on that silly idea!