Thanks for that. It says: "If the value of the products contained in your personal baggage
exceeds € 1,200:
• you can justify the «United Kingdom» origin of your
products by presenting a certificate (see trade and
cooperation agreement): no tax is due
• you cannot justify the «United Kingdom» origin: your
products will be taxed at the applicable rate."
So as long as the items in my luggage are worth under 1200 euros it's fine (how they will determine the value of a bag full of shirts and trousers I have no idea). BUT, even if the value exceeds 1200 euros, as long as I can demonstrate that the items are of UK origin then there is also no duty to pay. Now need to work out what the mysterious "certificate" is...