An update. Dropped car off at the dealer yesterday, they had it for a few hours and then called to say that they thought it was down to the non genuine towbar electrics (which was what I expected them to say, but pointed out that the voltage had dropped prior to the towbar being fitted). I was disappointed and thought I'd just get fobbed off, but credit to them they did a full software update, charged the original Volvo battery, fitted it, kept it overnight, checked it again today and they reckon the issue is solved.
I've just got it home and have checked the voltage - it's sitting at 12.4v. Will try again tomorrow and see if there's any significant drop. Fingers crossed it's sorted the issue as I really love the car.
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