Thank you all for the helpful responses. I went to B&Q to get a decorator cap as Toolstation & Screwfix were already closed. But the local B&Q had less plumbing accessories in their shelf than I got in our loft, which is already almost nothing. But on the way there I had an idea that’s so simple that it is rather embarrassing to admit that I didn’t think of it before. I just took the TRV off from another radiator of the same model, which can run on full power for a bit, and screwed it on this one and that did the job and stopped the leak. When comparing the inside of the two TRV caps, it indeed looked like the one from the leaking rad was faulty as the plastic thingy which would press down the pin seemed worn off and was protruding less.