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Thank you for the answer. We have contacted Volvo UK, they are 'confident' the garage will be able to fix it as they always do. They have helpfully extended the warranty for this issue, so that is one less thing to worry about. But still, the garage are perplexed and have now got Volvo Technical involved, so fingers crossed. The solutions they have tried (fuel pump replacements, both of them) have not fixed it, nor has a full software update. It seems to definitely be a technical issue.
The problem we have with the original garage is that they are the other side of the country, so we can't just take it back to them. I wouldn't know how to find out who the original owner was before us (though we do know that the garage we purchased it from had known the car from new). Our current local garage contacted them to ask if there were any issues, they said no (obviously as they couldn't admit to a known fault and I'm sure they won't have sold it with that knowledge, surely!?). It is just a waiting game now to find out if the garage can suss it out...