Atended the SHA anual ball at the weekend at the Spa, and I got asked the above question. My answer was yes, payback still looks like roughly 8 years. The pannels are providing a great deal of warm water at this time of year (though it has been cloudy lately) but no matter how many pannels you do have the temperature will not get much above 35 oc. Though this does save a lot of energy that would have been used to heat the water from 8oc (ground water temp at the moment) to 28oc (with a heat exchanger due to the specific heat capacity of water, you will normall only get the water to 8oc below the solar temperature).
I am still happy to say 6m3 of water at around 65oc from March to October on every bright day, plus with the pool heat exchanger the lower temperature of 35oc at this time of year is exchanged into the pool as the pool only needs to be around 30oc, very little heat is lost. This has worked so well that we have now taken out the now redundant second boiler to heat the pool water.