Problem with Wiring / Hysteresis after replacing Tank Stat

Hi There, this is my first post - sorry it's such a long one.

I have a Worcester Bosch 19/24CBi, and Potterton EP2000 programmer, a Potterton tank stat on the hot water tank, and 3 port HW/CH valve. A week ago it looked like the tank stat (PTT1) had failed, as we had no hot water, and no 'demand' light on PTT1.

My local plumbing supplier said the PTT1 had been replaced by PTT2. Bought one, and then found that the PTT2 expect 5 wires, but my old PTT1 only had 4. I made a note of the wiring in the PTT1

Checked this forum for previous problem, found a few posts, but nothing that really gave me the answer. I got it working (HW & CH), with a small amount of trial and error, but wasn't convinced my wiring was correct, so called Potterton, and they faxed me what was allegedly a wiring diagram for replacing PTT1 with Ptt2. However, it still showed 5 wires into the PTT2 !

However, it did show the Live from programmer to the TL on the PTT2, which I tried and this also seemed to work ok.

When wiring up the stat, there was a white plastic cover that held the wires and contacts in place in the backplate, which kept falling out, so whilst I was testing different wiring combinations, I left it out. After the system worked for several days, it was time to put it all back and tidy up, and this included putting the white plastic cover in place.

However, now it didn't work. Tried everything, but the only thing that worked, was removing the plastic cover. Conclusion is that the plastic cover is keeping something apart, preventing a connection? So, maybe the wiring is still wrong?

And now, in the morning, the tank stat is clicking rapidly (is this Hysteresis?). This only happens after we have drawn a bunch of hot water, the tank stat showed 'below' and was calling for heat, but the HW was off at the programmer. I turned the programmer to HW ON, and the clicking stopped.

Any ideas? Oh, and from looking in the site WIKI, it suggests tank stat should be at 60, but mine is set at 45, which is its lowest temp. Could this be part of it?

If you want HOT water then turn the stat up to 60.

The new PTT2 looks to be the electronic version so may require a neutral for the power requirements (?) hence the 5 wire.

So... turn up the heat! It must be that when you refit the plastic piece you're actually allowing the sensor to sense the cylinder temp therefore the cylinder must be above the setting of 45 thus telling the system the HW is hot enough, then as you pull the plastic bit out the sensor is away from the cyl so sensing ambient air temp. which is then BELOW the setting.

Refit as you say but turn it up to 60 degrees. Report back.

I can't seem to find a pdf of the cylinder stat via google, so can't comment on the terminals... though I'd expect the original C (common) to power the stat when the HW was switched on.