20 minutes of off peak sounds a bit weird. The off peak should come on for a total of 9 hours, and I think this is between 13:30 and 15:30 GMT, 10:00 to 12:00 GMT and 1:00 to 6:00 GMT or there abouts (assuming your on the economy 9 Tariff from EDF). One other problem I had in the past was that the thermostat was set too high and would trip out on the bottom element and I had to turn it down to 55/60 degree's ish to enable the unit to stay on. It certainly sounds to me as if the unit isn't staying on long enough to heat the tank. Especially as you have factored out the bleed valve.