Zanussi ZWD1662W & AEG Lavamat Turbo 16830 dryer problem

I recently purchased a Zanussi ZWD 1662 W washer/ dryer from Comets as a replacement from my trusty old Hoover wdm150 which had recently had bearing failure (although a very good reliable machine up to then). I chose this model as it had timed drying as opposed to sensor drying, and I had previously had problems with sensor drying (a long story which I wont go into here).
The first load completed washing and I then set it for an hours drying. After 45 minutes the dryer turned off and the clothes still being damp I restarted the dryer for another hour, but after 15 minutes it turned off again. I restarted it again, but this time watched the time left counter only to find that once the pump started after the first 5 minutes or so the timer started counting down rapidly at 20 second intervals instead of 1 minute intervals, the time reached about 28 minutes left (after about just 15 minutes) and then the counter jumped to zero and the machine switched off. I decided to follow this up with putting more wet items into the dryer to see if the drying cycle would then last longer, but again it switched off after just 15 minutes. I called out an engineer who confirmed there was a fault, although no fault code was found. I went back to comets who then sent out a replacement machine of the same model. Once the replacement machine was installed I put it onto a drying cycle for exactly the same problem to occur. It would run for 45 minutes on the first cycle and then only for around 15 minutes on subsequent cycles until it had completely cooled down again, no matter how long I set the timer for. I got another engineer out who confirmed there was a fault, but again, no fault code. Again, I went back to comets and requested that the exchange machine be a different model this time, I paid the extra and purchased an AEG Lavamat Turbo 16830 which comets assured me I would have no problem with, I got it home, installed and ran a wash cycle followed by a drying cycle. I set the dryer for sensor dry which this machine has but the clothes came out quite damp after it finished at around 45 minutes so I then switched it to timer dry and set it for an hour, but 15 minutes later it switched off. I switched it back on again and to my disbelief as I watched the timer it started counting down at 20 second intervals instead of 1 minute intervals as soon as the pump started, it reached around 28 minutes remaining then the timer jumped to zero and the machine switched off.
I could not believe that for a 3rd time (and with a different machine) I was again getting exactly the same problem The wash cycle works fine, this problem only occurs when drying.
I contacted AEG who told me that this problem should not occur and that If I set the machine for timed dry then it shoud stay switched on for that length of time and that there was obviously a fault. (I was also previously told that by Zanussi)
Again I called out the engineer who by now was finding it hard to believe that I could get 3 machines displaying exactly the same problem, but on this occasion actually witnessed the machine count down rapidly and switch off before the time had ended, and so started looking for an installation issue which he could not find. The water pressure is fine and it was suggested that I should get our power checked. I have now had this done and our power is fine.
I am at my wits end. I know washer/ dryers are not the best option, but unfortunately it is our only option as we have no space at all and we need a dryer as our son has an enuresis problem.
We have now been over 2 months without a working washer/ dryer in our house 🙁 🙁 🙁