Hi Mr Eagle,
I've now had time to follow up on your tip.
Immediately prior to any changes speedtest.net reported speeds of 17.99/7.75 down/up.
I then ran the optimiser program and the result on speedtest improved dramatically to 37.09/7.29.
Next I tried BT and it reported 36.2 which is similar to what it reported over the last couple of days.
So I suppose the conclusion is that the odd results appear to be an XP issue and that the BT site is more reliable under all conditions. Why I cannot even begin to guess unless they have some smart system that is measuring the timings at the modem/router rather than my PC.
Anyway, many thanks for the very useful tip.
Regards
BBN