It was not until after Monsieur David had left that we realised that, despite having hot water in the basin, we had none in the shower. It eventually occurred to us to remove the shower head, after which the hose produced piping hot water. The shower head was obviously the cause of the problem. We deduced that the holes had become partly blocked, thereby reducing the flow of water to below that needed to operate the gas heater in the boiler.
A trip to our favourite DIY store meant that a new shower head was easily obtained. I bought a rather flash one that could be adjusted to provide a gentle spray, or a body-tingling hard jet, or a combination of the two. There were cheaper ones on the shelf, but what the heck, I thought. It was a pity that we had to cut up the packaging, because it meant that I was unable to return the new shower head when we discovered that this one too reduced the flow of water to an unacceptable low.
It took the cheapest shower head on sale at the local supermarket to do the trick.