A bit of determination to fix it has fixed it! New pump is working well, and on Monday, I replaced the flexible tap connectors with normal 22mm piping - reducing all restrictions on flow, other than the actual heat exchanger itself. This means that the position of the H/exchanger is fixed, but it hasn't needed moving for a year.
I still believe that a low temperature collector is more effective for solar earth charging. They really only seem to do anything when there is direct sunshine. But when the sun shines and the tube circuit is not working, the tube manifold temperature rises to dangerous levels, like 135ºC, and I don't want an exploding steam engine in my loft!