Help diagnosing readouts and cause of no hot water

I've been experiencing intermittent failure of the hot water. It will work perfectly fine for 5 to 10 days, then for couple of days hot water just won't kick in, however long you run the taps for.

Typically if fails first thing in the morning when you least want to diagnose the issue, but I have kept the front panel off for the last few weeks so that I can try and diagnose the issue.

Here's a typical scenario when I am getting no hot water:

- Status
 - HT: bar lit
 - ST: bar lit
 - DHW: bar off
 - Msg: on
- Store T1
 - Current: 71 (can go much lower like 25)
 - T1 On: 71 or 74
 - T1 Off: 74 or 77
- PHE T2
 - Current: 17
 - T2 On: 70
 - T2 Off: 75
DHW T3
 - Current: H05 or another similar low value

The green LEDs, looking at my pics from the above scenario:
- 1 (Boiler): on
- 2 (Boiler pump): on
- 3 (Heating pump): on
- 4: off

However this morning when I had no hot water these were:
- 1 (Boiler): off
- 2 (Boiler pump): off
- 3 (Heating pump): on
- 4: off

I have also notice that the PHE pump (right hand pump) is not running when there is no hot water.

On some occasions, by switching to emergency HW for about 10 second, then switching back to normal operation, the pump kicks in and the water temperature starts to rise, however this evening when I tried this, the pump and temperature dropped as soon as I went back to normal operation.

Any advise on likely cause would be appreciated.

The PHE was replaced about 2 years ago, but most of the time since I have had a water softener in place - although that has been bypassed for a couple of weeks due to flow rates dropping as a result of the softener.

Check the pump.
And voltage at the pump.
Also is the unit full?
Mike