Hi all,
I ended up ringing customer support and after we went through the usual checks (driver updates, etc.) it was concluded that the laptop was faulty so I have to send it to Microsoft for a repair or replacement.
The support tech said the issue was arising equally on the 13 and 15" models and a repair seems to resolve the problem for 90% of customers and a replacement resolves the issue for 95% of people.
I'll organise the courier at the weekend and the process, including return delivery, is supposed to take no longer than 6 days.
I'll let you know how it goes.
Dan