I am dumbfounded by, and grateful for, all the replies and the effort taken to reply to my question. Many thanks to everyone.
Several interesting points have been made.
A smaller tank may help, but getting deliveries of less than 500 litres is difficult, expensive and almost impossible round here. A £13 water separator seems a cheaper solution than replacing the tank.
I think “best” has pin-pointed the real problem. Following your comments, I went and had a closer look at the filling hole. Sure enough, it is almost flush with the top surface of the tank. I was vaguely aware that it has a rubber seal,but on studying it closely, the seal is between the tank top surface and the flange or housing that the cap screws onto. The cap itself has no rubber seal, and just relies on the plastic to plastic interface to keep driving rain out. The cap is embossed “Cap must be closed tight”, but I had not realised why, and may have often left it slightly loose. I dip the tank about once a week, more often if I am running the boiler daily. The cap and its housing appear to have been designed specifically to encourage driving rain to get into the tank, with smooth curved surfaces and no ridges to interrupt the flow!
Furthermore, I live on a hillside in rains-every-day Wales, (Between Ammanford and Llandeilo – Wear the fox hat, as Prince Philip said to Queeny when she told him she was coming here to open our village hall.) with only open fields behind the tank. The wind fair whistles up the fields, driving the rain almost horizontally. Clearly I need a more protective cap to fit over the existing cap.
The rubber seal is well past its use-by date, so I shall be out for a tube of mastic tomorrow.
I shall probably give it all another try, with just the ToolStation water separator, or buy the £14 airline filter and try that, or buy the OFTEC one. I have searched OFTEC's Service and Commissioning, Installation, and Miscellaneous shops, but cannot find a water separator – any chance of a little more guidance on where to find it.