Quick update on this. Long story short turns out the problem was the alternator, which once my garage got to it was visibly damaged, as was the wiring harness (looks like high current had melted the insulation). Currently waiting for parts to arrive from Germany. Pretty big job to fix this - £2k+ for alternator and wiring harness and labour, luckily some of it is covered by a warranty. On the plus side I've got a brand new battery fitted so that job's sorted
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