In the absence of other takers, I'll try and explain it.
In many ways secondary double glazing is perfrectly satisfactory. It is just that people don't like it because it doesn't look great and you can't open the (outer) windows without fiddling with the secondary glazing.
Steaming up is caused by moisture in the air in your house being unable to remain as water vapour close to the colder DG surface. It condenses, because colder air is less able to retain the same volume of water vapour as invisible vapour.
As to why your secondary DG worked better than your new, I am struggling to understand. I think it might be due to the perspex inner - itself a better insulator than single glass. What is clear is that the inner surface temperature at the perspex surface face that you achieved must have been higher than that of your new DG.
To solve the problem, I would look very carefully at what causes water in your house, since any DG should be capable of preventing excessive condensation. Even in my well-insulated property, I small misty patch occurs along the bottom on a really cold window - but I assume you are talking dripping water?
So:
Are you using paraffin heaters?
Do you have a shower with an adequate extraction system to the outside?
Ditto question over the hob?
Do you dry the washing over rails in the house?
All of these reasons create large volumes of water vapour in the house. The best solution is to try and cut the amount of such vapour being created in the first place.